Ken you might want to check with a Mitel guy because I jumped ship when they took over. I'm guessing it has to do with Inter-Tel's affinity for the orange pair.
In older systems they would roll the orange pair. In the later systems that only required one pair to operate the digital phones, they used the orange pair and left the blue pair unused. The reason for this was that you could take a trunk directly to the jack on the blue pair and use the orange pair to power the digital phone. By doing this every jack in the place could be used for analog or digital. In this manner, the jack behind you could be a station, or just unplug the station and plug in a fax machine, or in a power failure just plug in a 2500 set and the access the trunk. It was really kind of nice.
I would think it'd be easier to fix them at the cross connect if you have many of them do change, and very few techs ever wired the blue pair to anything so they're probably wide open on the blocks. That said, I have heard the 8520 was far superior to most of the Mitel replacements.
