Agree with Gregory here.
Does this occur on every call or every odd call here and there?
You could always replicate the issue and check the streaming statistics of the call (Check Web Access is enabled on your CIPC) and look for incrementing sent / received packets during the entire call to see what your results are. Perhaps at the point where the audio stops, you will see the sent or received packet counters stop?
The streaming stats also tell you what source / destination IP's are used in the call, so that may reveal something about the issue.
