M12 AND MX10 DISCONTINUED!. 1) Try new batteries! The batteries usually last anywhere from 6 months to 2 years. 2) Check the compatibility settings. The unit must be set to #1 or #5! If it will not work on one of these numbers, but works on another number like #4, then the batteries are either dead or were installed incorrectly. Plantronics designed #4 as a battery test switch, if it works at all at that number, you need new batteries. Always remember that the M12 or M22 must always either be set to #5, #1, or, on very rare occasions, #2. 3) Check volume controls. There are three volume controls that can have an affect on the M12 or M22 sound quality. They are: the volume on the phone itself, the volume dial on top of the amp (affects the way they hear the caller), and the blue & white microphone screw located on the left hand side of the amp (often beneath a panel, affects the way the caller hears them). 4) Check setup. It is possible that the system was set up slightly incorrectly.…