Plantronics APN-91 Electronic Hook Switch Adapter (89280-11) Designed For: CS 510, 510 Spare Headset, 520, 520 Spare Headset, 520A, 530, 530A, 540, 540 Spare Headset, 540 Spare Headset C540A, 540A, 540-XD; Savi W710, W710/A, W710-M, W720, W720/A, W720-M, W730, W730/A, W730-M, W740, W740/A, W740/A-M, W740-M Product type: Electronic hook switch adapter Made by Plantronics Also compatible with Plantronics MDA200 Communications Hub and Voyager Legend CS Bluetooth Headset System