Huh, what are the chances.. I was going to post about the same problem in a different scenario.
I have a MCD 6.0 SP2 system sip trunked to a 3rd party IVR app. The IVR wants to make outbound calls to customers and give them information they requested. Obviously, it needs to know when the B party answers. I'm seeing the same thing as steve75 - for some unknown reason the Mitel is responding with 200 OK at about the time it receives a ring signal from the external telephone network. Basically, the IVR sends an INVITE, the Mitel replies with 100 Trying, then a few seconds later it sends 183 Session Progress, quickly followed by 200 OK. If we test to an extension, it works perfectly. The outbound trunks are E1 ISDN.
I've been all over my config, and have read a bunch of the sample configs for different providers on MOL, but I'm still in the dark. The only thing that looks like what I want is in Digital CO Trunk Circuit Descriptors there is a setting for Fake Answer Supervision After Outpulsing, but it doesn't apply to my trunk types as far as I can tell. It's like that setting is Yes on E1 trunks.
jrg0852 - I don't think this is timer related as I see it at random times roughly 3 to 6 seconds after INVITE.