all the handsets have done this? are they all on same digital module? we had digital modules that had trouble power supplies for a bit, half the unit stopped running, although can't remember what was on the screen? 'please wait ' etc...?
that fault was a long time ago now though...
also had a user plug a patch lead from a digital phone outlet in the wall into the back of their PC/ NIC.. that caused mayhem! random reboots.. then constant reboots.
Cheers,
Chris