The jack on the telephone is much too large for the port on the headset amplifier.
You might be connecting the Plantronics unit incorrectly. The line cord coming out of the wall (your telephone line) stays in your telephone. That is the larger sized jack.The hand receiver cable is the only cable to be removed from your telephone. Unplug the hand receiver from the base of the telephone, and connect it to the back of the Vista M12 (where it shows a picture of just a hand receiver).The short cable that came with the Vista M12 plugs into the back of the Vista M12 (where it shows a picture of a full telephone), and the other end plugs into your telephone where the hand receiver used to be.