# Troubleshooting analog phones and FXS ports with no dial tone or failed calls Canonical: https://warmtransfer.net/knowledge/analog-fxs-troubleshooting Last verified: 2026-10-02 When an analog phone on an FXS port gets no dial tone or will not register, documented causes include a disabled Profile Active setting on Grandstream HT801 V2 and HT802 V2 ATAs[^5], and extension-length or configuration-push problems on Cisco VG4xx gateways registered to Webex Calling[^7][^10]. For failed fax calls on these ports, the guidance covers T.38 settings, fax machine ECM and speed changes, and a recommendation to keep a dedicated PSTN line[^14][^8][^13][^12]. ## Confirming the port type and firmware On VG4xx gateways, FXO and ISDN interfaces are not currently supported for use with Webex Calling, and only FXS ports are[^9]. An analog device connected to an FXO or ISDN interface on a VG4xx is therefore outside what Webex Calling supports[^9]. Webex Calling's VG4xx article lists IOS XE 17.18.1a images as the supported firmware: vg400-universalk9, vg4x0-universalk9 for the VG410, and vg420-universalk9[^11]. The same article requires IOS XE 17.16.1 or later for site survivability[^11]. ## Diagnosing no dial tone On Grandstream HT801 V2 and HT802 V2 ATAs, a port with Profile Active disabled is not operational, and the analog phone connected to it gets no dial tone[^5]. The Grandstream HT801 V2 FXS port is specified for a 5 REN ring load up to 1 km on 24 AWG[^2]. The Grandstream HT802 V2 FXS ports are specified for a 2 REN ring load up to 1 km on 24 AWG[^3]. ## Diagnosing VG4xx ports that do not register A VG4xx port registers to Webex Calling only if the assigned user's or workspace's extension has at least 4 digits[^7]. If a port does not register after someone changed it while it was in use, Webex Calling's guidance is to push the configuration from Control Hub again once the port is idle[^10]. On a VG410 registered to Webex Calling, check whether the vDSP application shows Running state in `show app-hosting list`[^6]. If it does not, the documented fix is to run `voice vdsp remove` followed by `voice vdsp install`[^6]. ## Diagnosing failed fax calls Webex Calling advises switching an ATA fax port to T.38 to improve fax quality[^14]. Webex Calling also warns that faxes are not encrypted while T.38 is enabled, whereas the default setting encrypts fax data from the ATA to the Webex Calling service[^14]. For VG4xx ports connected to a fax machine on Webex Calling, the guidance is to enable T.38 so that it overrides the default audio codec[^8]. Grandstream HT80x V2 fax defaults to T.38 Group 3 fax relay at up to 14.4 kbps, and falls back automatically to G.711 fax pass-through[^4]. If ATA faxes still fail on Webex Calling, the guidance is to change the fax machine's ECM setting and lower its speed to 9600 or 2400 bps[^13]. For the best fax and data-device performance, the Webex Calling ATA fax article recommends continuing to use a dedicated PSTN line[^12]. ## See also - See also [Analog gateways ATA and paging devices](https://warmtransfer.net/knowledge/analog-gateways-ata). - See also [Setting up analog devices and ATAs on Webex Calling](https://warmtransfer.net/knowledge/webex-calling-analog-ata-setup). - See also [Fax and T.38 in cloud calling](https://warmtransfer.net/knowledge/fax-t38). - See also [Troubleshooting fax over IP failures](https://warmtransfer.net/knowledge/fax-over-ip-troubleshooting). - See also [POTS line replacement and copper retirement](https://warmtransfer.net/knowledge/pots-line-replacement). ## Applicability The VG4xx guidance lists IOS XE 17.18.1a as the supported firmware and requires IOS XE 17.16.1 or later for site survivability[^11]. Applies to: Cisco Webex Calling, Cisco VG410, Grandstream HT801 V2, and Grandstream HT802 V2. Deployments: on-premises and multi-tenant. Sources checked 2026-10-02. ## What remains uncertain The ring load and loop length that VG4xx FXS ports support are not covered by the sources below. Registration troubleshooting for Grandstream HT80x V2 ports, beyond the Profile Active setting, is not covered by the sources below. Whether the HT80x V2 automatic fallback to G.711 fax pass-through affects fax encryption on Webex Calling is not covered by the sources below. ## Sources [^1]: Because vendors' own fax guidance recommends a dedicated PSTN line for data devices and none of the reviewed ATA or VG documents addresses alarm or elevator lines, alarm dialers and elevator phones on FXS ports should be treated as unsupported until the device and monitoring vendors confirm compatibility (inferred). Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), article body - dedicated PSTN line note. Checked 2026-10-02. [^2]: The Grandstream HT801 V2 FXS port is specified for a 5 REN ring load up to 1 km on 24 AWG. Source: [HT80x V2 - Administration Guide](https://documentation.grandstream.com/knowledge-base/ht80x-v2-administration-guide/), technical specifications - Ring Load. Checked 2026-10-02. [^3]: The Grandstream HT802 V2 FXS ports are specified for a 2 REN ring load up to 1 km on 24 AWG. Source: [HT80x V2 - Administration Guide](https://documentation.grandstream.com/knowledge-base/ht80x-v2-administration-guide/), technical specifications - Ring Load. Checked 2026-10-02. [^4]: Grandstream HT80x V2 fax defaults to T.38 Group 3 fax relay up to 14.4 kbps with automatic fallback to G.711 fax pass-through. Source: [HT80x V2 - Administration Guide](https://documentation.grandstream.com/knowledge-base/ht80x-v2-administration-guide/), fax configuration - Fax Mode. Checked 2026-10-02. [^5]: On Grandstream HT801 V2 and HT802 V2 ATAs, if Profile Active is disabled for a port, the port is not operational and the analog phone gets no dial tone. Source: [HT80x V2 - Administration Guide](https://documentation.grandstream.com/knowledge-base/ht80x-v2-administration-guide/), FXS port settings - 'Profile Active' parameter. Checked 2026-10-02. [^6]: On a VG410 registered to Webex Calling, if the vDSP application is not in Running state in show app-hosting list, the fix given is voice vdsp remove followed by voice vdsp install. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), troubleshooting note on VG410 vDSP. Checked 2026-10-02. [^7]: For a VG4xx port to register to Webex Calling, the assigned user's or workspace's extension must have at least 4 digits. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), limitations list. Checked 2026-10-02. [^8]: For VG4xx ports connected to a fax machine on Webex Calling, the guidance is to enable T.38 to override the default audio codec. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), port configuration section - fax note. Checked 2026-10-02. [^9]: FXO and ISDN interfaces on VG4xx gateways are not currently supported for use with Webex Calling; only FXS ports are. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), article note on supported port types. Checked 2026-10-02. [^10]: If a VG4xx port does not register after it was changed while in use, Webex Calling guidance is to push the configuration from Control Hub again once the port is idle. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), troubleshooting note on port registration. Checked 2026-10-02. [^11]: Webex Calling's VG4xx article lists IOS XE 17.18.1a images (vg400-universalk9, vg4x0-universalk9 for VG410, vg420-universalk9) as the supported firmware, and requires IOS XE 17.16.1 or later for site survivability. Source: [Configure your Cisco Voice Gateway VG400, VG410, or VG420 in Control Hub](https://help.webex.com/en-us/article/nj4t084/Configure-your-Cisco-Voice-Gateway-VG400,-VG420,-or-VG410-in-Control-Hub), section on supported firmware versions. Checked 2026-10-02. [^12]: For best fax and data-device performance, the Webex Calling ATA fax article recommends continuing to use a dedicated PSTN line. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), article body - performance note. Checked 2026-10-02. [^13]: If ATA faxes still fail on Webex Calling, the guidance is to change the fax machine's ECM setting and lower its speed to 9600 or 2400 bps. Source: [Improve fax transmission with your ATA](https://help.webex.com/en-us/article/fzwl6ab/Improve-Fax-Transmission-with-Your-ATA), article body - 'What to do next'. Checked 2026-10-02. [^14]: Webex Calling advises switching an ATA fax port to T.38 to improve fax quality, but warns that with T.38 enabled faxes are not encrypted, whereas the default setting encrypts fax data 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 - 'Before you begin' and main procedure. Checked 2026-10-02.