I don't know if this will help or not - it is not mentioned in the manual - but on the right hand side of the base are four buttons labeled from the top: Find, Skip, Stop, Skip. If, when the phone is inactive and you are not playing a message, you press the bottom Skip button, you will get a message saying "message alert on" or message alert off". If you then press the button and hold it down for several seconds, it will reverse the setting. So if by any chance your phone at the moment is set to "message alert on", maybe reversing this will stop the perpetual bleep.
Jill, August 2008