This is a pet peeve of mine in respect to Nortel systems. If Avaya and Siemens can do this (and presumably others) on TDM systems, why can't Nortel?
There's a workaround IF your user has a multi-line phone. It's a two button solution, not one, but it more or less solves the problem and requires no programming on your part. When the call starts ringing, the user should press one of his OTHER line keys - one that is NOT ringing. This will give him dial tone. Then the user should press Goodbye, thus disconnecting the dial tone. That's two buttons. The effect of this action, however, is that the ringing call on the first line will stop its audible ringing! It will still flash on the phone set and finish its ring cycle, but the noise will stop. So it's a two button process: Line 2 - Goodbye. Problem solved. But obviously if a user only has a single line phone, there is no alternate line to access, and thus this solution won't work for them. I sent instructions on this to all my users with multiline phones and became a hero. Good luck!