BT SYNERGY 4500 PHONE - CALLERS CAN'T HEAR ME!?

My poor old Dad has a BT Synergy 4500 phone with 3 handsets, which have always worked well. Suddenly today callers can't hear him; it doesn't matter if he calls out or they call in, he can hear them but they can't hear him. I live 400 miles away from my Dad so can't diagnose the problem any further. Can you help, please? Annie