BT synergy 150 can't enter numbers on keypad?

We have a BT synergy 2150 - when you try to use the handset keypad it won't always accept all the numbers - after entering 4 or 5 numbers the keypad freezes until the phone is replaced in the cradle.

I have changed the battery and charged it fully but that hasn't solved the problem. Am now assuming the handset may be faulty