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Calls disconnect due to problems with disconnect supervision when Cisco CallManager Express FXO ports are connected to the Comcast VOIP over the cable system

Core Issue

The Cisco CallManager Express system is plugged into the Foreign Exchange Station (FXS) port of a Comcast cable system. The Comcast modem provides voice services.

The Comcast Analogue Telephone Adapter (ATA) built into the cable modem does not send any disconnect tones towards the customer premises equipment (CPE) device (Cisco CallManager Express router) until well over 60 seconds after the public switched telephone network (PSTN) side of the call has disconnected.

The tones Comcast ATA sends out are not recognized on the Cisco CallManager Express system, and voicemail ports and messages capacity are tied up with extended messages. These disconnect tones do match the correct North America tones and cadence of 480/620 hz , 500 msec on/500 msec off, and the Cisco IOS  supervisory disconnect tone disconnect feature is not able to recognize them as valid tones to enable the call to be dropped.


In order to resolve the issue, create a custom disconnect class for the voice port. This ideally now recognizes the relevant disconnect tones the Comcast device provides:

voice class custom-cptone Comcast
dualtone disconnect
frequency 480 625 

voice-port 0/2/0
supervisory disconnect dualtone mid-call

supervisory custom-cptone Comcast  
timeouts call-disconnect 5
timeouts wait-release 5

In regards to the time delay that the disconnect tone is sent to Cisco CallManager Express, the best thing to do is to contact Comcast (Service provider) and request them to reduce the time taken to playback the tones, and if necessary, use standard PSTN tones instead of the shortened duration ones they play through the ATA.

Refer to FXO Answer and Disconnect Supervision for more information.