This may occur if we don't get a response to our 'keep-alive' 

What happens is our switch sends a SIP 'keep-alive' after 50 minutes to make sure the call is still active. It does this by sending a SIP reINVITE for the live call, in which we expect a 200 OK response, that we then ACK back. If no response is received, the call is terminated by our switch.

We would have to investigate the call logs to confirm the above and if confirmed then the settings on your PBX as to why it is not responding to reINVITEs.

If you require this investigated further please open a support ticket via http://thinktel.distributel.happyfox.com/new/ or you can contact us by phone at 866-928-4465 Options 1-3-1