Keypad locked on telephone unable to make calls

Question

When dialing a telephone number the hadset will only allow the 1st 3 numbers to be enetered and then emits a continuos tone every time a key is depressed afterwards. 

 I suspect one of my children may have played with the handset and inadvertantly locked the keypad via a menu option. 

I have tried accessing several options on the menu that may support this and turning the feature off but with no success