Keypad problems

The keypad on my Nokia 7650 is not responding to being pressed during phonecalls. For example, after I call T-mobile, I'm asked to select an option from the menu. But when I press a key there is no response. I can dial numbers without any problems but during the call the keys are not registering. This is a problem that has only recently developed, it was fine up until a week ago. Has anyone else experienced this? How do I remedy it?