The gateway we were setting up was going inline of the PBX and PRI. The previous configuration was:
The new configuration is:
If you don't set the SIP Proxy IP address then everything will work fine except TDM to TDM routing for outbound calls. It took 6 hours to figure this out and it didn't make sense but the call flow for Nortel to PSTN resulted in the call being sent to OCS always. Calls from OCS to PSTN worked fine. Also, more maddening was that PSTN to Nortel worked fine.
So, remember......the quick installation guides are still a good reference for you and can save you a LOT of time and frustration for those key settings you may forget to change.