I have encountered an issue where on a SIP trunk between two Asterisk servers, only calls that are not answered, go to a non-standard RTP port and the Voicemail service is not handled by Asterisk
Example 1 - Everything Works
A Calls B, B answers. Good to go
Example 2 - No RTP in Voicemail
A calls B, B lets the call go to Voicemail. Voicemail Application activates, but no RTP is received.
After receiving RTP pakcets from the far side server the near side responds on a non-standard port. Shown in the trace below.
I have checked the rtp.conf and the start and end range match on both sides.
[general] prtpstart=10000 rtpend=20000
Anything else I should look for. I am looking at any firewall port forwarding or blocking, but I do not think thats the case here.