Thanks for the response.
Interesting that you mention that toll-free numbers can do this, this problem trunk happens to receive calls from the national telecoms provider here (Telkom SA) sourced from a toll-free number. The SIP trunk provider has ported that toll free state telecoms company number for us to a local trunk number with which we authenticate as a SIP peer. (Not sure if what Telkom here in South Africa define as “toll free” on a technical level match what you mention as “toll free” on an American / United States model.)
The thing is the number stays in the unanswered state (while in fact answered and working perfectly) for the entire duration of the call, so then it appears 200 OK is not received for the invite.
I therefore assume that the actual RTP negotiation process does NOT need the
200 OK for the invite, for the call itself is fine – it is just that Asterisk never “realises”, as regards the CDR, that the call was in fact answered.
At least it is working and audio flows back and forth.
I’ll see if I can come up with a SIP trace.