PRI would be the best way to go here I think, but you could also do plain T1 tie lines or even analog tie lines. FXO to FXS integration is also possible and easy to setup.
If you have some FXO and FXS ports on each system you can connect them to each other FXO<->FXS.
For example, if you go off hook on a FXO port of your CME you will get internal dial tone on the Nortel and can call extensions or PSTN the same as an an analog phone on that port. Same thing in the other direction applies- if a Nortel user dials a trunk group code for a group of FXO lines they can get dial tone and call an extension on your CME.
-- please remember to rate and mark answered helpful posts --