I've had this problem recently with a Uniden phone. I had to dissemble the phone and make a fine adjustment to the frequency potentiometer. This is usually toward the top of the circuit board and is adjusted by a straight edge screw driver. Mark the original position of the potentiometer with a marker, then turn it 180 degrees (choose clockwise or anti-clockwise, then if one doesn't work try the other!). You shouldn't need to go any further than 180 degrees but of course you can if you want.
Josh, September 2006