T41S units asking for last 5 digits of serial number then reseting to factory

[richard.vince]

I have several T41S and another model (a reception type Yealink model) that recently have been after powered up they have been asking for the last 5 digits of the serial number. Once this is entered into the phone via the keypad they reset back to factory defaults.
I have no idea why this is happening and it is driving me nuts. The phones are on 2 different sites and I have to travel to the other site to re-configure these phones after they have factory reset.