Help!! My BT Synergy 4500 answerphone with 4000 handset has died?

I have exactly the same problem with my Synergy 4500 phone as Mel Smith: Line is working, new good quality batteries are fully charged, displays on the handset are perfect, incoming call rings, but I can't answer calls, phone is dead. I can even dial out but again no sound at all. Help, please.

Doris Hiscox
March 2010