How can I fix my BT 4500 Cordless phone?

The calls list just shows 10 noughts (0000000) when the 'calls' button is pressed instead of the number of the last caller.

The phone has just suffered from a power failure and all cells had to be removed and mains power witched off for ten minutes prior to getting it working again. All contacts in the phone book have been wiped.

John Keeley.
July 2020