My BT Freestyle 710 handset is permanently handsfree?

Until recently it used to work properly (and is now out of warranty). We could turn handsfree on and off by pressing the green talk\handsfree button (button with green telephone and speaker symbols). But now it is always in handsfree mode. The full instruction guide for this phone does not cover this situation. I have tried resetting the hardware settings, but makes no difference. Anyone encountered this problem and found a solution?