For the past two nights my bedroom telephone has started bleeping (not ringing) at precisely 1 am. It does not stop when I lift the handset.,the bleep is coming from the phone base. It bleeps for up to a minute and then stops but last night it started again. I did "1471" in case someone was trying to get through but got a message "that number has not been recognised". The phone is about 14 years old. What is the cause?