Outbound calls on SIP trunk drop after 50-60 seconds

I’m hopeful that someone can point me in the right direction.

Inbound calls work perfectly, but ALL outbound calls on one of my SIP trunks get dropped after aprox. 50 - 60 seconds.

This does not happen when making calls through my ZAP trunk.

Any ideas where the problem might be?

Probably a NAT problem.

The Asterisk Server is in a DMZ currently, but I get the same result when I plug it right into my ADSL router with it’s own public IP address.

You say “one of my SIP trunks” - does that mean you’ve got accounts with more than one service provider and you only get the problem with one service provider? If so, it’s probably the service provider, not Asterisk.