you need to give more detail about what you have and what you are going to.
Is the nortel pbx going to a voip or tdm application?
Basically any cutover from one brand of pbx to another brand means lots of record gathering, more so, if they are both traditional pbx's. It means pulling and identifying every cross connect on the original pbx and cross referencing to the new pbx port number, what I call a tone cut (toning out every phone on cut day, easy to do on a small key system, but not so easy on a large pbx) is a very long process. programing wise you need to identify the present set up of each phone and make sure that it has the same type of features has before (boss to secretary arrangements, add-on key strips, ringing setups, call pickups groups, hunting and so on). analog phones are the hardest to identify, especially alarms and modems that are not used that often. Are the existing trunks ok as they are setup or do you have to get the dialtone provider to re-engineer them.
As I said you need to give a few details for a better answer.
