Asterisk 15-minute drops calls

This is a classic symptom of a NAT session timing out on a firewall. Options without changing your firewall:

• Look in the firewalls advanced firewall settings and see if you can see anything to do with session lifetime or expiry. Try extending this.
• See if your firewall has a SIP ALG in it anywhere. This may just be called VoIP mode with a tick box, or it could be as complex as 10 pages of configuration. Experiment with enabling/disabling that. ALGs can often have good/bad effects but it totally depends on the firewall and scenario.
• See if you can put the VoIP server on a routed rather than NAT'd network. Keep the firewall to shield it, but removing NAT entirely is a good cure for this. Some firewalls can this a DMZ zone, but be aware that the term DMZ in cheap SoHo firewalls can just mean 1-to-1 NAT with all ports open - this isn't what you want.

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