Hi what are the lines on the meridian ?
Basicly you have to intercept the incoming lines BEFORE they go to the Nortel.
The reason for this is that you will not need to make major changes to the Nortel when you migrate users. and finally the Nortel will wither and die with no users on it and it can simply be turned off.
I have used this method of migration with Asterisk and many other systems from 20 - 3000+ extensions over many years and always without issue.
DO NOT connect the Asterisk system after the Nortel IE to extensions or a spare ISDN trunk. The main reason for this is it means as number are to be migrated changes need to be made each time to the system and finally when the Nortel has no extensions you have to remove it from the middle which can be prone to issues.