Volume control and Mute switch for Microphone.ONLY WORKS with Plantronics M10, M12, M22 & MX10 Amplifiers or CISCO IP phone models : Cisco IP 6921, 6941, 6945, 6961, 7821, 7841, 7861 Cisco IP 7931G, 7940, 7940G, 7941, 7941G, 7942, 7942G, 7945, 7945G Cisco IP 7960, 7960G, 7961, 7961G, 7962, 7962G, 7965, 7965G Cisco IP 7970, 7970G, 7971, 7971G, 7975, 7975G Cisco IP 8811, 8841, 8851, 8861, 8941, 8945, 8961, 9951, 9971 or AVAYA CALLMASTER V VI ONLY COMPATIBLE FOR SOME SPECIFIC PHONE MODELS LISTED ABOVE. Not compatible with Cisco Spa phone series. FOR OTHER PHONE MODELS, PLS CONTACT US FOR COMPATIBILITY GUIDE OR ADVICE.