This is a Plantronics Savi/CS500 series APC-45 EHS cable for Cisco SPA512G, SPA514G, SPA525G, SPA525G2 Phone models - electronically takes your phone off hook, eliminating the need for the HL10 lifter. Only compatible with these Plantronics Savi and CS500 models:

Plantronics CS540, CS510, CS520, Plantronics Savi WO100, Plantronics Savi WO200, Plantronics Savi WO300, Plantronics Savi WO350, Plantronics Savi W740, Plantronics Savi W710, Plantronics Savi W720, Plantronics Savi W730, Plantronics Savi W745

This cable only works with the following Cisco IP Phones:

Cisco SPA512G, SPA514G,  SPA525G and SPA525G2 Phones

If you have other Cisco models:

Cisco IP 8961 9951 9971, you need the APU-72 instead

Cisco IP 7942G, 7945G, 7962G, 7965G, 7975G use APC-42  instead