FXO cards generating fax tone

Unanswered Question
Sep 19th, 2008


We have an FXO card in a 2811 router taking calls from the PSTN and routing them to Call Manager 5.1. Recently users are complaingng that random inbound calls are ringing to the phone as they should but that on some calls both parties can hear what they are describing as "fax machine sounds" when called party picks up. CM user can call them right back with no problems. Never an issue on outbound calls.

Anyone seen this before?

Thanks in advance! All replies rated!

I have this problem too.
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vmoopeung Thu, 09/25/2008 - 10:50

If the FXO supervisory disconnect tone feature is configured and the DSP detects a tone from the PSTN or PBX, the analog FXO port goes on-hook. You can configure the router's FXO voice port to detect supervisory disconnect tones before or after the call is answered.

Try upgrading IOS to version 12.3(8)T.

The following URL may help you:



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