What software version does the system run?
When the phone drops to version what does the station information MMC say?
There is a problem in the OS7200's that was fixed in 4.3 where digital extensions would change to be analogue therefore the phone would not work, the only fix was a download, default and re-upload of the programming.
Didn't affect all systems though.
I have never installed a 16trk, so can't help with the issue there