Fax and T.38 in cloud calling

See also

Sources

1. 1
   8x8 advises lowering fax speed to 9600 bps disabling ECM and using Standard resolution on fax machines connected through an ATA.

   Troubleshooting Issues with Fax Devices · Fax machine settings list · Checked 2026-09-24
2. 2
   8x8 Support will help with general fax machine settings but cannot troubleshoot the fax machine itself or a fax server once the ATA is confirmed working.

   Troubleshooting Issues with Fax Devices · Support scope paragraph · Checked 2026-09-24
3. 3
   Cisco's ATA 191/192 guide advises allocating at least 50 kbps for T.38 fax and approximately 100 kbps for G.711 fallback.

   Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services · Optimize Fax Completion Rates section · Checked 2026-09-24
4. 4
   Cisco's ATA 191/192 guide says that when a Cisco media gateway terminates to the PSTN, T.38 fax relay should be disabled and fax enabled via modem passthrough (fax protocol pass-through g711ulaw with fax rate disable) with a T.38 alternative given.

   Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services · Optimize Fax Completion Rates section; Cisco media gateway guidance · Checked 2026-09-24
5. 5
   On Cisco ATA 191/192 multiplatform firmware if a T.38 call cannot be set up the call automatically reverts to G.711 fallback.

   Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services · Optimize Fax Completion Rates section · Checked 2026-09-24
6. 6
   For fax on Cisco ATA 191 and 192 multiplatform firmware Cisco recommends G.711u (USA) or G.711a (elsewhere) as the preferred codec with silence suppression and echo cancellation disabled.

   Cisco ATA 191 and ATA 192 Analog Telephone Adapter Administration Guide for Multiplatform Firmware - Advanced Options for Phone Services · Optimize Fax Completion Rates section · Checked 2026-09-24
7. 7
   Cisco says the fax transport method must be consistent end to end across the originating gateway CUBE and terminating gateway path.

   CUBE Third-Party Interoperability Fax Guidelines · Background Information and CUBE Configuration sections · Checked 2026-09-24
8. 8
   For third-party interoperability Cisco recommends configuring CUBE with fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback none with dial-peer configuration taking precedence over global settings.

   CUBE Third-Party Interoperability Fax Guidelines · CUBE Configuration section · Checked 2026-09-24
9. 9
   Cisco states that NSE-based fax switchover is Cisco-proprietary so CUBE fax interworking with third-party equipment must use standards-based T.38 relay or protocol-based G.711 pass-through.

   CUBE Third-Party Interoperability Fax Guidelines · FoIP Transport Methods section · Checked 2026-09-24
10. 10
    Cisco's CUBE fax note says T.38 version 0 carries G3 fax speeds while T.38 version 3 supports Super G3 from Cisco IOS 15.1(1)T.

    CUBE Third-Party Interoperability Fax Guidelines · FoIP Transport Methods section; T.38 versions · Checked 2026-09-24
11. 11
    In CUBE fax call flows the terminating gateway detects the V.21 preamble and triggers switchover to T.38 with a SIP ReINVITE (or H.323 RequestMode) which CUBE must permit.

    CUBE Third-Party Interoperability Fax Guidelines · CUBE Fax Call Flows section · Checked 2026-09-24
12. 12
    As of 2026-09-24 the in-force edition of ITU-T T.38 is T.38 (11/15) and earlier editions from 06/98 onward are superseded.

    T.38 : Procedures for real-time Group 3 facsimile communication over IP networks · Recommendation version table; In force and Superseded rows · Checked 2026-09-24
13. 13
    ITU-T Recommendation T.38 is titled Procedures for real-time Group 3 facsimile communication over IP networks.

    T.38 : Procedures for real-time Group 3 facsimile communication over IP networks · Recommendation landing page title · Checked 2026-09-24
14. 14
    In the Webex Calling Local Gateway configuration for the registration-based trunk, T.38 fax transport is enabled by default.

    Configure Local Gateway on Cisco IOS XE for Webex Calling · Configure Webex Calling registration-based trunk section · Checked 2026-09-24
15. 15
    RingCentral requires a fax ATA to be connected to a DigitalLine product offering.

    Fax Set Up Analog Telephone Adapter ATA · Page body; requirements · Checked 2026-09-24
16. 16
    RingCentral states that T.38 is not guaranteed for every end-to-end connection and recommends testing speed quality and reliability before relying on ATA faxing.

    Fax Set Up Analog Telephone Adapter ATA · Page body; T.38 paragraph · Checked 2026-09-24
17. 17
    RingCentral's ATA fax article states that the service uses the T.38 standard for fax and adheres to FCC guidelines regarding its use.

    Fax Set Up Analog Telephone Adapter ATA · Page body; T.38 paragraph · Checked 2026-09-24
18. 18
    RFC 3362 registers the image/t38 media subtype for T.38 real-time fax used in SDP-based call setup and says the registration is not intended for email.

    Real-time Facsimile (T.38) - image/t38 MIME Sub-type Registration · Section 2 and Section 4 · Checked 2026-09-24
19. 19
    RFC 3362 notes that T.38 allows the use of either TCP or UDP depending on the service environment.

    Real-time Facsimile (T.38) - image/t38 MIME Sub-type Registration · Section 3 · Checked 2026-09-24
20. 20
    RFC 6913 defines the sip.fax media feature tag with values t38 and passthrough so SIP user agents can advertise fax capability in REGISTER and callers can request it with Accept-Contact in INVITE.

    Indicating Fax over IP Capability in the Session Initiation Protocol (SIP) · Section 4 Usage; Section 7 IANA Considerations · Checked 2026-09-24
21. 21
    RFC 6913 motivates the sip.fax tag by noting that fax calls are treated as voice calls until after they are connected which can route them to non-fax endpoints.

    Indicating Fax over IP Capability in the Session Initiation Protocol (SIP) · Section 3 Motivation · Checked 2026-09-24
22. 22
    RFC 7345 states that earlier secure fax mechanisms such as T.30 Annex H and T.38 over RTP/SAVP achieved little market adoption.

    UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS) · Section 1 Introduction · Checked 2026-09-24
23. 23
    RFC 7345 defines transport of UDPTL (the predominant T.38 transport) over DTLS negotiated in SDP with the proto value UDP/TLS/UDPTL.

    UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS) · Section 1 Introduction; Section 4 SDP Offerer/Answerer Procedures · Checked 2026-09-24
24. 24
    To connect analog devices to Microsoft Teams Direct Routing an ATA is required and the ATA must be supported by the certified SBC vendor.

    Direct Routing - Connecting analog devices · Introduction paragraph · Checked 2026-09-24
25. 25
    Microsoft's Direct Routing analog devices guidance counts fax machines among analog devices alongside analog phones and overhead pagers.

    Direct Routing - Connecting analog devices · Considerations section; first bullet · Checked 2026-09-24
26. 26
    Because Microsoft's Direct Routing analog guidance says nothing about T.38 and defers ATA and SBC configuration to the SBC vendors, fax media handling in Direct Routing appears to be the responsibility of the ATA and SBC rather than a documented Teams capability.inferred

    Direct Routing - Connecting analog devices · Example section; SBC manufacturer documentation links · Checked 2026-09-24
27. 27
    Cloud calling vendors diverge on ATA fax transport: Webex Calling guidance points toward enabling T.38 and RingCentral says it uses T.38 while Zoom's documented OBi setup disables T.38 in favour of G.711 passthrough.inferred

    Setting up faxing (Poly OBi series ATA) · Codec settings step; compared with cisco-help-fzwl6ab-improve-fax-ata T.38 guidance and ringcentral-fax-set-up-ata T.38 paragraph · Checked 2026-09-24
28. 28
    Local Gateway for Webex for Government does not support Fax (T.38).

    Configure Local Gateway on Cisco IOS XE for Webex Calling · Local Gateway for Webex for Government section · Checked 2026-09-24
29. 29
    Fax (T.38) is listed among the features not supported in Webex for Government Calling.

    Webex for Government Calling · Features not supported in Webex for Government · Checked 2026-09-24
30. 30
    The 488 tech note presupposes that T.38 mode is already configured in Control Hub on the fax device.

    Fix 488 SIP Error for Fax T.38 ReINVITE Rejected by Webex Calling · Prerequisites or problem context · Checked 2026-09-24
31. 31
    After changing compression to Normal for the 488 T.38 issue, Cisco says the fax device must be restarted for the change to apply.

    Fix 488 SIP Error for Fax T.38 ReINVITE Rejected by Webex Calling · Solution section; final step · Checked 2026-09-24
32. 32
    Cisco's fix for the Webex Calling T.38 ReINVITE 488 error is to set the fax user's or workspace's Call Experience compression to Normal compression in Control Hub on the Calling tab.

    Fix 488 SIP Error for Fax T.38 ReINVITE Rejected by Webex Calling · Solution section; steps 1 to 5 · Checked 2026-09-24
33. 33
    A documented Webex Calling fax failure is that the T.38 switchover ReINVITE from a registered fax device is answered with SIP 488 Not Acceptable Here so T.38 is never negotiated.

    Fix 488 SIP Error for Fax T.38 ReINVITE Rejected by Webex Calling · Problem section · Checked 2026-09-24
34. 34
    If Webex Calling ATA fax problems persist, Cisco advises lowering the fax machine baud rate to 9600 or 2400 bps.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; troubleshooting steps · Checked 2026-09-24
35. 35
    Cisco's Webex Calling ATA fax troubleshooting steps include adjusting Error Correction Mode settings and reducing the number of pages per transmission.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; troubleshooting steps · Checked 2026-09-24
36. 36
    Cisco advises setting the fax machine behind a Webex Calling ATA to Fax mode or Auto Answer Fax mode so incoming calls are not treated as voice calls.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; fax machine configuration · Checked 2026-09-24
37. 37
    Cisco notes that keeping a dedicated PSTN line remains an option for best fax and modem performance instead of faxing through a Webex Calling ATA.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; introduction · Checked 2026-09-24
38. 38
    Cisco states that when T.38 is enabled on a Webex Calling ATA, faxes are not encrypted.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; T.38 mode guidance · Checked 2026-09-24
39. 39
    In Webex Calling ATA fax guidance, T.38 mode is disabled by default.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; T.38 mode guidance · Checked 2026-09-24
40. 40
    Cisco's Webex Calling help article on ATA fax transmission says that enabling T.38 mode can improve fax quality through an ATA.

    Improve fax transmission with your ATA · Improve fax transmission with your ATA; T.38 mode guidance · Checked 2026-09-24
41. 41
    For Webex Calling ATA fax the documented choice is a trade-off between the encrypted default path and T.38 which improves reliability but is unencrypted; no Webex source found documents RFC 7345 secure T.38.inferred

    Improve fax transmission with your ATA · T.38 mode guidance; compared with rfc-7345-udptl-over-dtls Section 1 · Checked 2026-09-24
42. 42
    Webex Calling Fax Messaging extends the user messaging service so users can receive store review and manage fax messages with notifications similar to voicemail.

    Configure Fax Message Feature for Webex Calling Users · Background Information section · Checked 2026-09-24
43. 43
    Fax Messaging for a Webex Calling user is enabled in Control Hub under the user's Calling voicemail settings with the Receive Fax Messages toggle and a fax phone number and requires the user's voicemail to be enabled.

    Configure Fax Message Feature for Webex Calling Users · Prerequisites and Configure sections · Checked 2026-09-24
44. 44
    Webex Calling Fax Messaging delivers received faxes in TIFF image format retrievable by email or from the User Portal voicemail section.

    Configure Fax Message Feature for Webex Calling Users · Verify section; retrieval methods · Checked 2026-09-24
45. 45
    Before unassigning the phone number of a Webex Calling voicemail group with fax enabled the administrator must disable fax messaging.

    Manage a shared voicemail and inbound fax box for Webex Calling · Delete or edit a voicemail group section · Checked 2026-09-24
46. 46
    A Webex Calling voicemail group needs a separate full phone number for fax service while a number is optional for voicemail only.

    Manage a shared voicemail and inbound fax box for Webex Calling · Create or edit a voicemail group; phone number field · Checked 2026-09-24
47. 47
    A Webex Calling voicemail group can be set to receive fax messages.

    Manage a shared voicemail and inbound fax box for Webex Calling · Create or edit a voicemail group; fax message option · Checked 2026-09-24
48. 48
    Zoom's AudioCodes MP1XX fax setup sets Fax Transport Mode and the V.21 to V.34 modem transport types to Events Only with a G.711 fax/modem bypass coder and supports only FXS ports.

    Setting up faxing (AudioCodes MP1XX series) · Prerequisites; Configuration > VoIP > Media > Fax/Modem/CID Settings step · Checked 2026-09-24
49. 49
    Zoom says fax over VoIP has technology-based industry-wide limitations and recommends that businesses dependent on regular multi-page faxing keep an analog line and a traditional fax machine.

    Setting up faxing (AudioCodes MP1XX series) · Introductory note · Checked 2026-09-24
50. 50
    Zoom's OBi fax guidance tells users to lower fax speed (to no less than 9600 bps) and disable Error Correction Mode and compression on the fax machine.

    Setting up faxing (Poly OBi series ATA) · Fax machine settings step · Checked 2026-09-24
51. 51
    Zoom documents fax setup for the Poly OBi30X OBi508 and ATA40X analog telephone adapters on Zoom Phone.

    Setting up faxing (Poly OBi series ATA) · Prerequisites and title · Checked 2026-09-24
52. 52
    Zoom's Poly OBi fax setup for Zoom Phone sets T38Enable to Disabled and FaxPassThroughCodec to G711U (North America) or G711A so fax uses G.711 passthrough rather than T.38.

    Setting up faxing (Poly OBi series ATA) · Codec settings step · Checked 2026-09-24

Documents

Cite this page

WarmTransfer. (2026, September 24). Fax and T.38 in cloud calling. WarmTransfer. https://warmtransfer.net/knowledge/fax-t38

@misc{warmtransfer-fax-t38,
  title  = {Fax and T.38 in cloud calling},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/fax-t38},
  note   = {Verified 2026-09-24}
}