It sounds like you're doing exactly what i'm doing! (not that i've got it working, you understand!)
I can get OCS to send SIP "stuff" to the Mitel but when i trace in Ethereal I can see that the mediation server receives a "SIP/2.0 404 Not Found" from the Mitel's address.
I pretty sure it's a mitel config problem and not OCS.
Also, can you "Play On Phone" using Exchange 2k7?
That doesn't work for me. Using Exchange, I have managed to get a Mitel phone to ring but when i answer I can't hear anything.
btw - What does your next-hop connections page look like on the OCS Med. ?
Thanks,
Thad
[EDIT] ps... Our Mitel listens (TCP) on Port 5070, which I wasn't expecting. You might want to check that in "System IP Port Assignment"