I'd also ask if they either have current networking background or have an interest in such. Since VoIP is becoming more and popular I'd almost prefer a person with knowledge of networking and next to no experience with Nortel.
Then ask yourself a question prior to the interviews. Do you know the Nortel side well? Any one can learn the little grey box if they have a great teacher that knows their stuff.
Another question would be scenario types. Ex. There's an issue, not a big user impacting one, but an issue none the less. Not knowing the answer, what/where would you go for help? Sites like this one are awesome and testing one's ability to use all resources around them including this site could make or break a person.
Another favorite of mine is asking the person to describe their most stressful outage/issue and have them tell you their troubleshooting steps. If you get a very detailed answer that includes all the little steps needed, that may be a person to look at twice. If you get well I called my vendor and waited on them, you may want to put the lure back in water to see what other fish you can catch, lol
Prior to my jobs working on Nortel, I had outside plant experience (POTS install and cable repair) and I was lucky to have a boss that hired me Nortel green. She showed me what she knew, perhaps grooming me, and I haven't looked back since. If you can be the grooming type, anyone with the initiative, who works well under stress is trainable.