Thanks to everyone who responded.
Here is how it I made this work. An in-house PRI between two Norstars requires a T1 Cross-Over cable. You must use the Sl-1 protocol with one side set as a master and the other as a slave, and then you must not have any numbers which overlap if you are going to use the desination code routing for 4 digits. After you change all of this, then when you place a call that is internal to the network the call goes over the PRI and uses the target lines assigned to the phone on the other system.
I did have some success using T-1 instead of PRI, and setting some channels as E&M and others as DID's, but I couldn't consistently make this function.
I don't know about the software level support of certain releases of software which has been suggested, but this setup is using 4.1 on both systems and using the PRI it has worked flawlessly. I didn't want to upgrade as both system have older Application Module Mail systems, and some higher releases of Nortel Norstar software are incompatible with lower releases of mail software.