# Troubleshooting fax over IP failures

Canonical: https://warmtransfer.net/knowledge/fax-over-ip-troubleshooting

Last verified: 2026-10-02

RFC 6913 describes SIP fax calls as normally set up as voice calls and switched to T.38 only after answer, so they can be routed to user agents that are not fax capable[^40]. Across a Cisco VoIP path, every device must use the same fax protocol and the same switchover mechanism, and a Cisco device cannot transcode a T.38 leg into a G.711 pass-through leg mid-path[^41][^27].

## How the switchover works

RFC 6913 defines the `sip.fax` media feature tag with the values `t38`, meaning fax will use ITU-T T.38, and `passthrough`, meaning fax will be carried in G.711[^39]. RFC 6913 says a calling user agent placing a fax call SHOULD include the `sip.fax` tag in the Accept-Contact header of the INVITE[^38]. RFC 3362 registers the `image/t38` media type, defined by ITU-T T.38, for use as an SDP media descriptor, and notes that the registration is not intended for email[^37].

A T.38 re-INVITE carries an `m=image` media line that specifies `udptl t38`[^35]. That media line has attributes such as `T38FaxVersion`, `T38FaxRateManagement` (`transferredTCF`), `T38FaxMaxBuffer`, `T38FaxMaxDatagram` and `T38FaxUdpEC` (`t38UDPRedundancy`)[^35].

On Cisco IOS voice gateways that use protocol-based T.38, the terminating gateway sends the re-INVITE when it detects the V.21 preamble flags from the answering fax[^54]. Cisco Named Signaling Events (NSE) fax switchover is Cisco-proprietary and cannot be used toward third-party SIP providers[^33]. Toward those providers, standards-based (protocol-based) T.38 switchover must be used instead[^33].

Bandwidth's T.38 service requires the call to be set up as a G.711 VoIP call, followed by a customer re-INVITE to T.38 that must be offered and accepted[^14]. For outbound faxes, Telnyx by default sends the T.38 re-INVITE itself once it detects fax tone, and the initiator can be set to Telnyx, Customer, or Disabled[^51]. For inbound faxes, Telnyx expects the customer side to send the T.38 re-INVITE[^50]. Unchecking Enable T38 FAX gateway makes Telnyx refuse T.38 re-INVITEs, so the call stays on G.711[^50]. WarmTransfer's reading of the sources is that the side expected to send the re-INVITE varies by carrier and direction, so a re-INVITE-side mismatch is a plausible cause of calls that never leave G.711[^36].

See also [Fax and T.38 in cloud calling](https://warmtransfer.net/knowledge/fax-t38).

## Diagnosing it

Cisco's fax relay methodology starts by confirming that ordinary voice calls complete over the same path before investigating fax[^53]. Cisco also advises confirming that the fax machines work with each other across the PSTN without traversing the VoIP network, which separates terminal problems from network problems[^52].

On Cisco gateways, `show call active fax brief` identifies which dial peers a fax call matched[^42]. `show voice dsp` confirms whether the fax codec loaded in the DSPs[^42]. Cisco's fax troubleshooting debugs include `debug fax relay t30` (all-level-1 or all), `debug ccsip`, `debug voice ccapi inout` and `debug voip vtsp all`[^21].

## SIP 488 on the T.38 re-INVITE

If CUBE rejects a T.38 re-INVITE with SIP 488, Cisco advises checking that T.38 is configured globally and on both the incoming and outgoing dial-peers[^1]. Cisco then advises using `debug ccsip` to see why SIP rejected the re-INVITE[^1]. On Cisco IOS, a `fax protocol t38` setting on a VoIP dial-peer takes precedence over the global `voice service voip` setting[^22].

The Cisco IOS `fax protocol t38` command offers the fallback modes `cisco`, `none`, or `pass-through` with `g711ulaw` or `g711alaw`, used when T.38 cannot be negotiated at fax time[^45]. In the classic Cisco IOS command reference, the command defaults to `ls-redundancy 0 hs-redundancy 0 fallback cisco`[^43]. On the AS5350, AS5400 and AS5850, the fallback default is `none`[^43].

Bandwidth states that if the T.38 re-INVITE is unsuccessful, a SIP 488 Not Acceptable Here is sent and transmission should fall back to G.711[^13].

See also [SIP 488 and codec negotiation failures](https://warmtransfer.net/knowledge/sip-488-codec-failures).

## Training failures, packet loss and mid-page failures

In Cisco fax relay debugs, bad CRC errors on received T.30 frames indicate corrupted signals and lead to Failure To Train (FTT) responses[^12]. When a fax relay call shows FTT, Cisco advises checking T1/E1 digital interfaces for clocking errors and slips[^29].

With fax relay, ECM has a low tolerance for jitter and packet loss, approximately 2 percent[^24]. Disabling ECM on a Cisco fax relay dial-peer enables fax relay packet loss concealment, which tolerates about 10 percent packet loss[^26]. Setting T.38 `hs-redundancy` above 0 on Cisco IOS significantly increases the network bandwidth the fax call consumes[^30].

We infer that a fax that trains and then fails partway through a page most plausibly points to packet loss or jitter on the IP path, for which the documented mitigations are disabling ECM, lowering fax speed and adding T.38 redundancy[^32].

For fax pass-through testing on Cisco gateways, echo cancellation and voice activity detection should be disabled (`no echo-cancel enable`, `no vad`) because they degrade modulated fax signals[^34]. On Cisco gateways, the `fax rate voice` setting restricts the fax rate to the bandwidth of the configured voice codec[^28]. An explicit value such as `fax rate 14400` overrides that limit[^28].

## Super G3 and ECM

ECM is optional for G3 fax but mandatory for Super G3 (V.34) fax[^25]. Cisco documentation from 2015 states that Cisco gateways support only G3 fax over T.38[^19]. On an SG3 call, it says, the originating gateway squelches the V.8 CM tone so the SG3 machines fall back to G3[^19]. T.38 version 3 (Super G3 speeds) is supported on Cisco IOS from release 15.1(1)T onward[^46]. How these two statements apply to current IOS XE releases is not covered by the sources below. On the Cisco ATA 191 with Unified CM, V.34 is not supported for T.38 relay fax[^10].

## Platform differences

### Cisco ATA 191

On Unified CM, the Cisco ATA 191 fax mode is set on the phone configuration page[^11]. The available modes are Fax passthrough, T.38 fax relay, NSE Fax passthrough G711ulaw, and NSE Fax passthrough G711alaw[^11].

For the ATA 191 Multiplatform line configuration, Cisco recommends the following fax settings[^7][^5][^6].

- FAX Passthru Method set to ReINVITE[^7].
- G.711u (USA) or G.711a (elsewhere) as the preferred codec, with Use pref. codec only set to Yes[^5].
- Silence suppression and echo cancellation off[^5].
- Network Jitter Level set to very high and Jitter Buffer Adjustment set to no[^6].
- Call waiting and three-way calling disabled on the fax line[^6].

Cisco's ATA 191 Multiplatform fax troubleshooting advises confirming fax machine speeds between 7200 and 14400 bps[^8]. It also advises checking network jitter, loss and delay, and testing between 2 ATAs in a controlled environment[^8].

### Webex Calling ATAs

By default, Webex Calling ATAs send fax with T.38 mode disabled, and the fax data is encrypted from the ATA to the Webex Calling service[^56]. When T.38 is enabled on a Webex Calling ATA, the faxes are not encrypted[^59]. If problems persist after T.38 is enabled, Cisco suggests lowering the fax machine baud rate to 9600 or 2400 bps, adjusting ECM, and sending fewer pages per call[^58]. Cisco cautions that fax machines and modems may not function optimally through a Webex Calling ATA and recommends a dedicated PSTN line for best results[^57].

See also [Setting up analog devices and ATAs on Webex Calling](https://warmtransfer.net/knowledge/webex-calling-analog-ata-setup).

### Teams Direct Routing

Microsoft's Direct Routing guidance counts fax machines as analog devices, which connect to Direct Routing through an analog telephony adapter behind the SBC[^48]. The analog telephony adapter must be supported by the certified SBC vendor[^47]. The SBC routes analog-device calls to Teams or the PSTN according to its own routing table[^47].

See also [Troubleshooting Teams Direct Routing](https://warmtransfer.net/knowledge/teams-direct-routing-troubleshooting).

### Asterisk

In Asterisk `res_pjsip`, T.38 UDPTL support on an endpoint is off by default (`t38_udptl = no`)[^4]. `t38_udptl_ec` selects UDPTL error correction as `none` (the default), `fec` or `redundancy`[^3]. `t38_udptl_maxdatagram` defaults to 0 and overrides the maximum datagram size advertised by a remote endpoint; it is intended for broken endpoints[^2].

### Carrier recommendations

Bandwidth states that fax will not work over compressed codecs and recommends uncompressed G.711u for G.711 faxing[^17]. Bandwidth recommends setting the T.38 MaxDatagram to 176[^16]. Bandwidth offers no SLA or guarantee on fax performance parameters[^18]. Bandwidth also does not support troubleshooting deep into RTP or UDPTL data[^18]. Telnyx recommends disabling ECM on the fax machine and setting the baud rate to 9600 or below for faxing over its network[^49].

## Applicability

Applies to: Cisco IOS voice gateways, Cisco Unified Border Element, Cisco IOS, Cisco ATA 191, Cisco ATA 191 Multiplatform, Cisco Webex Calling, Microsoft Teams Phone Direct Routing, Sangoma Asterisk, Bandwidth SIP trunking, and Telnyx SIP trunking. Deployments: on-premises and multi-tenant. Sources checked 2026-10-02. T.38 version 3 support applies to Cisco IOS from release 15.1(1)T onward[^46]. The `fax protocol t38` defaults described here come from the classic Cisco IOS command reference, which gives a different fallback default for the AS5350, AS5400 and AS5850[^43]. The in-force edition of ITU-T Recommendation T.38 is T.38 (11/15), approved November 2015[^31]. Earlier editions, from 06/98 through 09/10, are superseded[^31]. The Cisco ATA 191 guidance covers both Unified CM firmware and Multiplatform firmware, and the recommended settings differ between them[^11][^7].

## What remains uncertain

Which side initiates the T.38 re-INVITE on Webex Calling ATAs or through a Teams Direct Routing SBC is not covered by the sources below. Whether Teams Direct Routing carries T.38 end to end is not covered by the sources below. WarmTransfer's reading of the sources is that a re-INVITE-side mismatch between carrier and customer is a plausible cause of calls that stay on G.711, and this should be confirmed against signaling traces[^36]. We infer that mid-page failures after successful training point to packet loss or jitter on the IP path, and this should be confirmed with network measurements[^32].

## Sources

[^1]: For a T.38 re-INVITE rejected with SIP 488 on CUBE, Cisco advises checking that T.38 is configured globally and on both the incoming and outgoing dial-peers, then using debug ccsip to see why SIP rejected the re-INVITE. Source: [CUBE Third-Party Interoperability Fax Guidelines](https://www.cisco.com/c/en/us/support/docs/voice/t38/116280-configure-cube-00.html), Troubleshoot section (488 Not Acceptable Media). Checked 2026-10-02.
[^2]: Asterisk res_pjsip t38_udptl_maxdatagram (default 0) overrides the maximum datagram size advertised by a remote endpoint, intended for broken endpoints. Source: [res_pjsip - Asterisk Documentation (Asterisk 21)](https://docs.asterisk.org/Asterisk_21_Documentation/API_Documentation/Module_Configuration/res_pjsip/), endpoint configuration option t38_udptl_maxdatagram. Checked 2026-10-02.
[^3]: Asterisk res_pjsip t38_udptl_ec selects UDPTL error correction as none (default), fec or redundancy. Source: [res_pjsip - Asterisk Documentation (Asterisk 21)](https://docs.asterisk.org/Asterisk_21_Documentation/API_Documentation/Module_Configuration/res_pjsip/), endpoint configuration option t38_udptl_ec. Checked 2026-10-02.
[^4]: In Asterisk res_pjsip, T.38 UDPTL support on an endpoint is off by default (t38_udptl = no). Source: [res_pjsip - Asterisk Documentation (Asterisk 21)](https://docs.asterisk.org/Asterisk_21_Documentation/API_Documentation/Module_Configuration/res_pjsip/), endpoint configuration option t38_udptl. Checked 2026-10-02.
[^5]: For fax, Cisco recommends the ATA 191 Multiplatform line use G.711u (USA) or G.711a (elsewhere) as preferred codec with Use pref. codec only set to Yes, and silence suppression and echo cancellation off. Source: [Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/19x/3PCC/english/admin-guide/at9x_b_ata191-192-admin-mp/at9x_b_ata191-192-admin-mp_chapter_01000.html), Optimize Fax Completion Rates. Checked 2026-10-02.
[^6]: For fax, Cisco recommends setting Network Jitter Level to very high and Jitter Buffer Adjustment to no on the ATA 191 Multiplatform, and disabling call waiting and three-way calling on the fax line. Source: [Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/19x/3PCC/english/admin-guide/at9x_b_ata191-192-admin-mp/at9x_b_ata191-192-admin-mp_chapter_01000.html), Optimize Fax Completion Rates. Checked 2026-10-02.
[^7]: For fax, Cisco recommends setting FAX Passthru Method to ReINVITE on the ATA 191 Multiplatform line configuration. Source: [Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/19x/3PCC/english/admin-guide/at9x_b_ata191-192-admin-mp/at9x_b_ata191-192-admin-mp_chapter_01000.html), Optimize Fax Completion Rates. Checked 2026-10-02.
[^8]: Cisco's ATA 191 Multiplatform fax troubleshooting advises confirming fax machine speeds between 7200 and 14400 bps, checking network jitter, loss and delay, and testing between two ATAs in a controlled environment. Source: [Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/19x/3PCC/english/admin-guide/at9x_b_ata191-192-admin-mp/at9x_b_ata191-192-admin-mp_chapter_01000.html), Optimize Fax Completion Rates / fax troubleshooting steps. Checked 2026-10-02.
[^9]: The ATA 191/192 Multiplatform guide states that if a T.38 call cannot be set up, the call automatically reverts to G.711 fallback. Source: [Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/19x/3PCC/english/admin-guide/at9x_b_ata191-192-admin-mp/at9x_b_ata191-192-admin-mp_chapter_01000.html), Optimize Fax Completion Rates, gateway note. Checked 2026-10-02.
[^10]: On the Cisco ATA 191 with Unified CM, V.34 is not supported for T.38 relay fax. Source: [Cisco ATA 191 Analog Telephone Adapter Administration Guide for Cisco Unified Communications Manager - Configure Fax Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/191/english/admin-guide/at91_b_ata191-admin-guide/at91_b_ata191-admin-guide_chapter_0100.html), Configure Fax Services, supported modem standards note. Checked 2026-10-02.
[^11]: On Unified CM, the Cisco ATA 191 fax mode is chosen on the phone configuration page from Fax passthrough, T.38 fax relay, NSE Fax passthrough G711ulaw, or NSE Fax passthrough G711alaw. Source: [Cisco ATA 191 Analog Telephone Adapter Administration Guide for Cisco Unified Communications Manager - Configure Fax Services](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cata/191/english/admin-guide/at91_b_ata191-admin-guide/at91_b_ata191-admin-guide_chapter_0100.html), Configure Fax Services > Fax Mode. Checked 2026-10-02.
[^12]: In Cisco fax relay debugs, bad CRC errors on received T.30 frames indicate corrupted signals and lead to Failure To Train (FTT) responses. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), debug fax relay t30 output discussion. Checked 2026-10-02.
[^13]: Bandwidth states that if the T.38 re-INVITE is unsuccessful a SIP 488 Not Acceptable Here is sent and transmission should fall back to G.711. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), T.38 support section. Checked 2026-10-02.
[^14]: Bandwidth's T.38 service requires the call to be set up as a G.711 VoIP call, followed by a customer re-INVITE to T.38 that must be offered and accepted. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), T.38 support section. Checked 2026-10-02.
[^15]: For G.711 faxing, Bandwidth says ECM is not supported and speeds are limited to 9600 or 14.4k, and recommends setting the fax machine to its lowest baud rate with error correction mode off. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), Recommended guidelines for VoIP faxing. Checked 2026-10-02.
[^16]: Bandwidth recommends setting the T.38 MaxDatagram to 176. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), Recommended guidelines for VoIP faxing. Checked 2026-10-02.
[^17]: Bandwidth states that fax will not work over compressed codecs and recommends uncompressed G.711u for G.711 faxing. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), Recommended guidelines for VoIP faxing. Checked 2026-10-02.
[^18]: Bandwidth offers no SLA or guarantee on fax performance parameters and does not support troubleshooting deep into RTP or UDPTL data. Source: [Bandwidth T.38 faxing support guide](https://www.bandwidth.com/support/en/articles/12822959-bandwidth-t-38-faxing-support-guide), T.38 support section. Checked 2026-10-02.
[^19]: Cisco gateways support only G3 fax over T.38; on an SG3 call the originating gateway squelches the V.8 CM tone so the SG3 machines fall back to G3. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), SG3 fax over T.38 section. Checked 2026-10-02.
[^20]: In Cisco topologies that include Unified CM, the T.38 switchover first sends a T.38 offer with a 0.0.0.0 address and later another T.38 re-INVITE with the real IP address. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), T.38 re-INVITE SDP discussion, CUCM-involved topologies note. Checked 2026-10-02.
[^21]: Cisco's fax troubleshooting debugs include debug fax relay t30 (all-level-1 or all), debug ccsip, debug voice ccapi inout and debug voip vtsp all. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), Debug commands section. Checked 2026-10-02.
[^22]: On Cisco IOS, a fax protocol t38 setting on a VoIP dial-peer takes precedence over the global voice service voip setting. Source: [Cisco IOS Voice Command Reference - F](https://www.cisco.com/c/en/us/td/docs/ios/voice/command/reference/vr_book/vr_f1.html), fax protocol t38 (dial peer), Usage Guidelines. Checked 2026-10-02.
[^23]: Cisco fax relay disables ECM by having the DSP firmware modify the T.30 Digital Identification Signal (DIS) so the machines do not negotiate ECM. Source: [Cisco IOS Voice Command Reference - F](https://www.cisco.com/c/en/us/td/docs/ios/voice/command/reference/vr_book/vr_f1.html), fax-relay ecm-disable, Usage Guidelines. Checked 2026-10-02.
[^24]: With fax relay, ECM has a low tolerance for jitter and packet loss, approximately two percent. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), ECM / packet loss section. Checked 2026-10-02.
[^25]: ECM is optional for G3 fax but mandatory for Super G3 (V.34) fax. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), Fax background / SG3 section. Checked 2026-10-02.
[^26]: Disabling ECM on a Cisco fax relay dial-peer enables fax relay packet loss concealment, which tolerates about ten percent packet loss. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), ECM / packet loss section. Checked 2026-10-02.
[^27]: Unlike audio, fax protocols cannot be transcoded, so a T.38 leg cannot be converted to a G.711 pass-through leg mid-path by a Cisco device. Source: [Commonly Supported Fax/Modem Call Flows](https://www.cisco.com/c/en/us/support/docs/voice/t38/215863-commonly-supported-fax-modem-call-flows.html), Introduction / background on fax call flows. Checked 2026-10-02.
[^28]: On Cisco gateways the fax rate voice setting restricts the fax rate to the bandwidth of the configured voice codec; an explicit value such as fax rate 14400 overrides that limit. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), Fax rate section. Checked 2026-10-02.
[^29]: When a fax relay call shows Failure To Train (FTT), Cisco advises checking T1/E1 digital interfaces for clocking errors and slips. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), Troubleshoot section, TCF/FTT. Checked 2026-10-02.
[^30]: Setting T.38 hs-redundancy above 0 on Cisco IOS significantly increases the network bandwidth the fax call consumes. Source: [Cisco IOS Voice Command Reference - F](https://www.cisco.com/c/en/us/td/docs/ios/voice/command/reference/vr_book/vr_f1.html), fax protocol t38 (dial peer), Usage Guidelines / caution. Checked 2026-10-02.
[^31]: The in-force edition of ITU-T Recommendation T.38 is T.38 (11/15), approved November 2015; earlier editions from 06/98 through 09/10 are superseded. Source: [T.38 : Procedures for real-time Group 3 facsimile communication over IP networks](https://www.itu.int/rec/T-REC-T.38/en), Recommendation edition table. Checked 2026-10-02.
[^32]: A fax that trains and then fails partway through a page most plausibly points to packet loss or jitter on the IP path, for which the documented mitigations are disabling ECM, lowering fax speed and adding T.38 redundancy (inferred). Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), ECM / packet loss section, read with cisco-help-fzwl6ab-improve-fax-ata troubleshooting list. Checked 2026-10-02.
[^33]: Cisco Named Signaling Events (NSE) fax switchover is Cisco-proprietary and cannot be used toward third-party SIP providers; standards-based (protocol-based) T.38 switchover must be used instead. Source: [CUBE Third-Party Interoperability Fax Guidelines](https://www.cisco.com/c/en/us/support/docs/voice/t38/116280-configure-cube-00.html), Configure section, note on NSE. Checked 2026-10-02.
[^34]: For fax pass-through testing on Cisco gateways, echo cancellation and voice activity detection should be disabled (no echo-cancel enable, no vad) because they degrade modulated fax signals. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), Fax pass-through section. Checked 2026-10-02.
[^35]: A T.38 re-INVITE carries an m=image ... udptl t38 media line with attributes such as T38FaxVersion, T38FaxRateManagement (transferredTCF), T38FaxMaxBuffer, T38FaxMaxDatagram and T38FaxUdpEC (t38UDPRedundancy). Source: [CUBE Third-Party Interoperability Fax Guidelines](https://www.cisco.com/c/en/us/support/docs/voice/t38/116280-configure-cube-00.html), Example T.38 ReINVITE SDP trace. Checked 2026-10-02.
[^36]: Which side sends the T.38 re-INVITE differs by carrier and direction (Bandwidth expects a customer re-INVITE; Telnyx defaults to sending it on outbound and expects the customer to send it on inbound), so a re-INVITE-side mismatch is a plausible cause of calls that never leave G.711 (inferred). Source: [Fax service with Telnyx (via T.38 or G711)](https://support.telnyx.com/en/articles/1130672-fax-service-with-telnyx-via-t-38-or-g711), Outbound and inbound fax sections, read against bandwidth-support-t38-faxing-guide T.38 support section. Checked 2026-10-02.
[^37]: RFC 3362 registers the image/t38 media type, defined by ITU-T T.38, for use as an SDP media descriptor, and notes the registration is not intended for email. Source: [Real-time Facsimile (T.38) - image/t38 MIME Sub-type Registration](https://www.rfc-editor.org/rfc/rfc3362.html), Abstract; Section 3.1. Checked 2026-10-02.
[^38]: RFC 6913 says a calling user agent placing a fax call SHOULD include the sip.fax media feature tag in the Accept-Contact header of the INVITE. Source: [Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc6913.html), Section 4. Checked 2026-10-02.
[^39]: RFC 6913 defines the sip.fax media feature tag with values t38 (fax will use ITU-T T.38) and passthrough (fax will be carried in G.711). Source: [Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc6913.html), Section 4. Checked 2026-10-02.
[^40]: RFC 6913 describes that SIP fax calls are normally set up as voice calls and switched to T.38 only after answer, so they can be routed to user agents that are not fax capable. Source: [Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc6913.html), Abstract and Section 1 (Introduction). Checked 2026-10-02.
[^41]: For a fax call to succeed across a Cisco VoIP path, every device on the VoIP path must use the same fax protocol and the same switchover mechanism (NSE or protocol-based). Source: [Commonly Supported Fax/Modem Call Flows](https://www.cisco.com/c/en/us/support/docs/voice/t38/215863-commonly-supported-fax-modem-call-flows.html), Background section on fax protocol and switchover matching. Checked 2026-10-02.
[^42]: show call active fax brief identifies which dial peers a fax call matched, and show voice dsp confirms whether the fax codec loaded in the DSPs. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), Verify / show commands section. Checked 2026-10-02.
[^43]: In the classic Cisco IOS command reference, fax protocol t38 defaults to ls-redundancy 0 hs-redundancy 0 fallback cisco, except on the AS5350, AS5400 and AS5850 where the fallback default is none. Source: [Cisco IOS Voice Command Reference - F](https://www.cisco.com/c/en/us/td/docs/ios/voice/command/reference/vr_book/vr_f1.html), fax protocol t38 (dial peer), Command Default. Checked 2026-10-02.
[^44]: Cisco's Fax-SIP troubleshooting guide gives network limits for T.38 of delay under 1000 ms and jitter under 300 ms. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), Network requirements / recommendations section. Checked 2026-10-02.
[^45]: The Cisco IOS fax protocol t38 command offers fallback modes cisco, none, or pass-through g711ulaw or g711alaw, used when T.38 cannot be negotiated at fax time. Source: [Cisco IOS Voice Command Reference - F](https://www.cisco.com/c/en/us/td/docs/ios/voice/command/reference/vr_book/vr_f1.html), fax protocol t38 (dial peer), Syntax Description. Checked 2026-10-02.
[^46]: T.38 version 3 (Super G3 speeds) is supported on Cisco IOS from release 15.1(1)T onward. Source: [CUBE Third-Party Interoperability Fax Guidelines](https://www.cisco.com/c/en/us/support/docs/voice/t38/116280-configure-cube-00.html), Configure section, T.38 version note. Checked 2026-10-02.
[^47]: For Teams Direct Routing, the analog telephony adapter must be supported by the certified SBC vendor, and the SBC routes analog-device calls to Teams or the PSTN according to its own routing table. Source: [Direct Routing - Connecting analog devices](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-analog-devices), Introduction paragraphs. Checked 2026-10-02.
[^48]: Microsoft's Direct Routing guidance counts fax machines as analog devices, which connect to Direct Routing through an analog telephony adapter behind the SBC. Source: [Direct Routing - Connecting analog devices](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-analog-devices), Introduction; Considerations section. Checked 2026-10-02.
[^49]: Telnyx recommends disabling ECM on the fax machine and setting the baud rate to 9600 or below for faxing over its network. Source: [Fax service with Telnyx (via T.38 or G711)](https://support.telnyx.com/en/articles/1130672-fax-service-with-telnyx-via-t-38-or-g711), Recommended fax machine settings. Checked 2026-10-02.
[^50]: For inbound faxes, Telnyx expects the customer side to send the T.38 re-INVITE; unchecking Enable T38 FAX gateway makes Telnyx refuse T.38 re-INVITEs so the call stays on G.711. Source: [Fax service with Telnyx (via T.38 or G711)](https://support.telnyx.com/en/articles/1130672-fax-service-with-telnyx-via-t-38-or-g711), Inbound fax section; Enable T38 FAX gateway option. Checked 2026-10-02.
[^51]: For outbound faxes, Telnyx by default sends the T.38 re-INVITE itself once fax tone is detected; the initiator can be set to Telnyx, Customer, or Disabled. Source: [Fax service with Telnyx (via T.38 or G711)](https://support.telnyx.com/en/articles/1130672-fax-service-with-telnyx-via-t-38-or-g711), Outbound fax / T.38 re-INVITE settings. Checked 2026-10-02.
[^52]: Cisco advises verifying that the fax machines work with each other across the PSTN without traversing the VoIP network, to isolate terminal problems from network problems. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), Troubleshooting methodology, isolate the problem. Checked 2026-10-02.
[^53]: Cisco's fax relay methodology starts by confirming that ordinary voice calls complete over the same path before investigating fax. Source: [Troubleshoot Fax Relay](https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html), Troubleshooting methodology, first steps. Checked 2026-10-02.
[^54]: On Cisco IOS voice gateways using protocol-based T.38, the terminating gateway initiates the switchover from voice to T.38 by sending the re-INVITE once it detects the V.21 preamble flags from the answering fax. Source: [Fax-SIP Troubleshoot Guide](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html), T.38 fax relay call flow section (TGW switchover on V.21 flag). Checked 2026-10-02.
[^55]: Cisco's 2013 CUBE third-party fax guidelines state that Verizon, although claiming T.38 over SIP support, never initiates a voice-to-T.38 switchover when it is the terminating gateway. Source: [CUBE Third-Party Interoperability Fax Guidelines](https://www.cisco.com/c/en/us/support/docs/voice/t38/116280-configure-cube-00.html), Verizon interoperability note within the T.38 configuration discussion. Checked 2026-10-02.
[^56]: By default, Webex Calling ATAs send fax with T.38 mode disabled, and the fax data is encrypted from the ATA to the Webex Calling service. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), Article body, opening section. Checked 2026-10-02.
[^57]: Cisco cautions that fax machines and modems may not function optimally through a Webex Calling ATA and recommends a dedicated PSTN line for best results. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), Device limitations note. Checked 2026-10-02.
[^58]: If Webex Calling ATA fax problems persist after enabling T.38, Cisco suggests lowering the fax machine baud rate to 9600 or 2400 bps, adjusting ECM, and sending fewer pages per call. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), Troubleshooting list. Checked 2026-10-02.
[^59]: When T.38 is enabled on a Webex Calling ATA, the faxes are not encrypted. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), Article body, T.38 note. Checked 2026-10-02.