I am not a Nortel man, but I recently had a similar problem with a Vodavi Starplus System.
I had the operator extension programmed like I had all of the other phones for Forward to Voice Mail on Busy or No Answer. For a regular station user, this is fine, but for someone that might receive calls reguarly while on another line, that is not good.
What happens is that when she is not on a call and a call comes in, she has time to answer, or the system picks up on the 4th ring. When she is on another call, the forwarding automatically goes to voice mail and answers the call immediately.
I corrected the issue by forwarding her calls to voice mail for No Answer only, not for Busy calls.
Perhaps the Nortel has the same options for station forwarding.
Hope this helps.
Gary Blake