I have an Asterisk server (15.5, FreePBX) with three SIP trunks from different providers configured, two of them are working fine while the third for every call keep sendind the invite despite the correct answer from the trunk.
The trunk was working well until we had some NAT problem with a change of configuration in the firewall, but I solved this issue and now I can see that the server connects to the trunk with the right ip and I can also see the answers coming to the right interface, whit sngrep this is what a call looks like:
After the invite the trunk reply with 100, but then the server resends the invite and never responds to trunk's messages. The call is correctly placed (and billed) by the provider but we are unable to talk as RTP channel is never opened. What could be the reason for this behavior?