BT Studio 1000 not ansering incoming call?

I have a BT Studio 1500 base unit and two 1000 handsets. One of the handsets can't answer incoming calls - an incoming call causes all three handsets to ring, 2 display the caller's number but the problem handset just displays "CALL". When I try and engage the incoming call on the problem handset I just hear a tone and the other handsets continue to ring. The problem handset can make external calls, and can anser internal calls - it's just incoming external calls that are the problem. I've tried swapping batteries, swapping phones onto different base units etc - nothing helps.