NORTEL CS1K <-> Asterisk

We are trying to integrate with an affiliate company’s VOIP implementation. They are using NORTEL CS1K, and we are running Asterisk 1.4 with FreePBX globally. I easily set up a SIP trunk that can dial into the NORTEL system, and calls flow from Asterisk to NORTEL without issue. However, every time the NORTEL system attempts a connection to Asterisk, the call is listed as rejected, but nothing appears on the Asterisk console.

Are there any special tips or tricks to getting this working?

Many thanks,