We have a Polycom for our conference room, when a call hits 30 mins on the dot the external line disconnects but the polycom still shows the active call.
I have made calls from other IP phones using the same trunk just fine.
In the Server Activity Log, make a call, wait until in it hangs, the check the logs, at least to see which side is sending the BYE message, then maybe you can work towards the correct direction.