I have a weird issue with calling from the Cisco Call Manager 4.0 over a SIP trunk to a Cisco 7040 with SIP firmware.
Weird is that I can create a call if I dial a Call Manager extension from that SIP 7940 and it works without problems,
but if I dial from a Call Manager the extension of the SIP 7940 then the Call Manager sends out an INVITE, then accepts the RINGING, but then after a call is accepted on a 7940, CCM just ignores the "SIP/2.0 200 OK" messages generated by the 7940.
If I replace the Cisco SIP 7940 with some SIP softphone then it works in both ways.
I attached a dump of SIP messages between a 3rd party SIP server and Call Manager.
The Cisco 7940 with a SIP firmware is registered to a 3rd party SIP Server.
7940 is running on P0S3-08-2-00.
BTW: If I dial that SIP phone from a Cisco 3745 voice gateway then it works.