intermittent audio on FXS port to door-entry system

Unanswered Question
Jan 2nd, 2008

We have an issue with an analog door entry system connected to FXS port on a cisco 2821 ISR running a Cisco CallManager Express IPT solution. This problem is in the UK.

The door system behaves (or is supposed to) like an analog phone and can be used to call an extension or just dial a default number. The signalling all appears to be OK and internal IP phones ring, but 70% of the time there is no audio to or from the door phone.

The FXS port configuration and some additional is attached. I also have some debugs which may help.

I have this problem too.
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bwilmoth Tue, 01/08/2008 - 06:43

Check to see if a firewall or NAT is in the path of the Real-Time Protocol (RTP) packets. By default, firewalls and NATs cause one-way audio or no audio. Cisco IOS and PIX NATs and firewalls have the ability to modify those connections so that two-way audio can flow.

blockdgold Tue, 01/08/2008 - 09:05

thanks for the response. Note this is an intermittant problem - wouldn't a firewall issue cause an problem 100% of the time? Have checked the ACLs and allowed all voice traffic between vlans on the router.

Are there any other known issues with analog phones on FXS ports?

gsidhu Sat, 10/24/2009 - 13:05


I have client with same requirement. Did this problem get resolved?



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