Keypad problem:

Does your phone lock up, beep or constant ring? Do the lights seem to work fine but no buttons will work? Does the phone pick up a CO line by itself? If any one of these things happen, the keypad may be going bad and must be replaced. It will only get worse.

The problem explained:

The Old style AT&T Partner MLS series phones (MLS-6, MLS-12, MLS-18) and the Merlin Legend (MLX-10, MLX-16, MLX-20, MLX-28) have a very intermittent keypad problem. There are dozens of circuit tracings that cross each other on the keypad that have a thin layer of electrical insulation between them. Sometimes the insulation was not enough and under the right conditions they can short out and the phone thinks a button is being pushed. When this happens either the phone will do things on its own or no keys will work on the phone at all. This condition is effected by temperature and humidity and can be VERY difficult to troubleshoot.