If memory serves Me right, the 4406, 4412, 4424, and 4450 were no longer supported by Avaya as of R7. They probably do work, but if there is an issue with those phones You will be on Your own.
I have replaced a lot of 44xx phones, and external DS modules due to failures! When 44xx phones go bad, they usually short out the module.
The best bet would be to upgrade their 44xx phones, it will save you a headache in the long run.
Andre