Avaya made 4400, 4406, 4412, 4424 Phones, originally intended for Merlin Magix, they went EOS Aug 2009.
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.
The 4400 & 6402 have been blocked since R3 ( It never worked right on R2.1 & below anyway )
the 4424LD version of the 4424 is not supported at all, and is blocked since R4
Also 44xx only work on expansion modules, they are blocked on internal Digital 8 modules on the 500 & 500v2.
These Phones take a lot of juice. most module failures I have seen in the field were filled with mostly 4412/4424 Phones.
Their firmware is not upgradable, and only have one choice of ringtone when used on IPO.
I do NOT recommend using these Phones on IPO, it will save you a headache in the long run.
just My 2 cents.