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.