No audio after 10 seconds on inbound calls

[miken1957]

I have just moved from Trixbox to PiaF and i have a problem with all calls losing audio after 15 seconds of being answered by my PiaF IVR's. i have not changed or upgraded my hardware and the install went well (or so i thought). However, calls to different trunks being answered by different ivr's all drop audio (not the call) after approx 15 seconds. As the hardware is the same and Trixbox worked fine I am at a loss as to why the audio should go down like this on every call without exception. Has anyone any experience of this problem, and if so, a fix for it?
Apologies if this has been asked and answered b4 and I have missed it.
Thx in advance, Mike