There seems to be a hard limit on incoming calls set a certain way- a telephone engineer told me about this, but I don't know the specifics. When on a call, the moment the call hits 15 minutes it drops. Does anyone know the specifics of this behaviour, and how to change it?
This is a classic symptom of a NAT session timing out on a firewall. Options without changing your firewall:
I had this issue yesterday with 1.6.x branch (stock in Debian Squeeze), it's due to faulty SIP session timers. The resolution is to use 1.8.x
In my case this was a simple dist-upgrade as Wheezy is now frozen and contains the 1.8.x branch.
https://issues.asterisk.org/jira/browse/ASTERISK-18996