I have a Nortel IP Phone 2002 working with asterisk with chan_unistm. Calls can be made and received on the phone no problem. The issue is with the display, when I am dialing I do not see the numbers being dialed. I have seen some talk about the display size being an issue with the 2002 phones. I have seen some stuff about patches but I am not sure which to use or if they still work.
you will need to locate the patch and place it in the correct directory, then once the patch is applied re-compile asterisk with the new patch in place and do the old make / make install routine and you should be good.