I have 3 X Beocom 6000 hand sets connected to one base, which have been in use continually for 5 years or more without problems. One hand set has deregistered by itself and will not re-register despite going through all the required steps. It finds the base on search, identifies it correctly, the correct pin is entered (I have checked this with the other handsets), but then shows "error" and "not registered" with the result that you get no dialling tone and are unable to make/receive calls from this handset. Strangely all the phonebook, redial and last call information are shown on the handset in question. Please advise thank you