BT Freelance XD 5500 Trio?

How do we stop the answer machine message switching on and off by it's self as if the button is being pressed? It keeps saying "answer of, answer on, Hello your call canot be taken at the moment... (and the rest of the standard message) as if there is a caller that has rang, but there is no call. There is also bepping as if the buttons are being pressed. This has been happening for days and i would love to know how to stop this from happening! The phone wasn't new but it has been working fine for the last couple of months now... Please help!