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Walkthrough Wednesdays

FXO ports are locked - Supervisory Disconnect Tone not working on hairpinned (POTS to POTS) calls


Core Issue

If an inwards voice call through a Foreign Exchange Office (FXO) port is hairpinnined back out another FXO port on one of these voice cards (new generation NM-HD-2V, NM-HD-2VE, NM-HD-HDV2 voice modules), the Supervisory Disconnect Tone feature does not work correctly. This may be due to an IP phone being call forwarded to an external number or a direct transfer from an IP phone to external number.

If a call comes through the voice port and is directed to a Voice over IP (VoIP) device (IP  phone, voice mail, etc.), the disconnect tone feature functions correctly.

The new generation voice cards have a Time-Division Multiplexing (TDM)-based architecture. The Digital Signal Processors (DSPs) are dropped out of call paths if the call is hairpinned between two ports on the same network module, since technically they are no longer required. Therefore, the Pulse Code Modulation (PCM) audio stream from one port is connected directly to the other port.

As a result, the Supervisory Disconnect Tone feature is not active, since there  is no DSP monitoring the audio streams for the configured disconnect tones.


To resolve the issue, add this configuration:

voice-card 1

  no local-bypass


This command changes the default behaviour of the voice card and keeps the  DSP across the audio stream for the duration of the call, even if the call  is later connected directly to another voice port on the same network  module. Therefore, the Supervisory Disconnect Tone feature is active  as expected.

In order to speed up the disconnection if the Supervisory Disconnect Tone feature is used, issue these commands to the voice port configuration:

timeouts call-disconnect 5
timeouts wait-release 5

These settings shorten the time the router waits before it determines that the call needs to be dropped due to disconnect tones  played by the network. The normal Cisco IOS  defaults are 30 and 60 seconds respectively, so these reduce the drop time to 5 seconds:

For further information on Supervisory Disconnect Tone problems, refer to these documents:

Voice Gateway Hardware Components




Voice modules