Plantronics M12 Vista adapter amplifier:

M12 Vista Universal Amplifier connects Plantronics headset to just about any modular single or multi-line phone and offer ergonomically designed volume, headset/handset, and mute controls. Connects Plantronics headsets to most single or multi-line telephones

 
Plantronics M12 Vista adapter amplifier features:

       With its easy-to-use one-button selection of headset or handset, Vista amplifiers deliver precise levels of listening and talking comfort, superior protection, and excellent audio performance to your office telephone.

       Vista ™ Universal Amplifiers connect Plantronics headsets to just about any modular single- or multi-line phone and offer ergonomically designed volume, headset/handset, and mute controls.

       Select between your headset and handset at the touch of a button.

       Includes 2 AA Batteries, 1 10ft. QD Cable to mod jack, 1 short mod cable

       Separate controls allow independent adjustment of outgoing and incoming volume.