I found the brochure for the nortel IP phones on their website and it states that their phones have a built in layer 2 switch "that splits the network catagory 5 cable into seperate feeds providing an additional RJ-45 port to connect your PC."
They call this hardware based priority. It sounds like they are using the unused pairs in the cat5 cable as a seperate physical link. I have not seen what stabndard they are using for that, I would suggest contacting Nortel support to find out or sniff the traffic coming from the phone to see what it's doing. Hope this helps....