ONLY Compatible with Plantronics M10, M12, M22 & MX10 Amplifiers or CISCO IP phone models : CTS 500 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