Has anybody tried using Aastra SIP phones on the 8100/9100?

A customer purchased the sets from the cable provider a couple years ago who have been providing a hosted setup. Cable company was recently purchased, and the new company says that those phones will be incompatible in the new year and wants them to buy new phones.

We could either sign them up with another hosted provider or possibly install 9100 with third party SIP licenses. Not sure what features the Aastra sets would have, if any or if the 9100 would see them more like an analog extension (no line appearances or DSS keys)