SIP protocol has not been set. Nortel uses a proprietary SIP signaling.
I have attempted to use a few providers here in the states. The issue seems to be the SIP response codes. I was able to make/receive calls, but had issue with post connection DTMF pass-thru. Could not choose options from Auto attendant, enter CC# or bank info ect...
When standardization is complete, I think Nortel will have no choice but to make changes. As for now, SIP on a Nortel is BCM to BCM only