Asterisk "Re-transmission timeout" errors

To avoid Re-transmission timeout errors on public sip trunks:

1. on the firewall forward udp ports 5060, 10001-20000 to the internal ip address of your pbx
2. on the firewall turn off any sip alg or sip transformations in the firewall
3. either disable or set firewall udp timers to 3 minutes or more
4. make sure your pbx has a static ip address and in the freepbx sip settings set NAT to yes
5. turn on NAT on in your firewall
6. if you have a static public ip address use it with your sip provider instead of registration