Troubleshooting analog phones and FXS ports with no dial tone or failed calls

Verified 2026-10-02 · 14 sources · tier 2

Also known as analog-phone-no-dial-tone, fxs-port-no-dial-tone.

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.
• See also Setting up analog devices and ATAs on Webex Calling.
• See also Fax and T.38 in cloud calling.
• See also Troubleshooting fax over IP failures.
• See also POTS line replacement and copper retirement.

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.

WarmTransfer. (2026, October 2). Troubleshooting analog phones and FXS ports with no dial tone or failed calls. WarmTransfer. https://warmtransfer.net/knowledge/analog-fxs-troubleshooting

@misc{warmtransfer-analog-fxs-troubleshooting,
  title  = {Troubleshooting analog phones and FXS ports with no dial tone or failed calls},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/analog-fxs-troubleshooting},
  note   = {Verified 2026-10-02}
}