US Cable Headset Adapter for 2.5mm Headset, RJ9/RJ10/RJ22 Headset Plug Adapter Converter for Aastra Shoretel Cisco E20 Polycom 335 VVX400 Digium D40 D70 Altigen Telephone Phones Convert 2.5mm plug to single 4P4C RJ9/RJ10 headset or handset plug for your office phone Save money by buying standard phone headsets to use with your phone's headset jack Reduce clutter with single headset Does not work with RJ11 jack, which is wider than your handset or headset jack (6 positions vs. 4). RJ11 jack is what is in the wall. RJ9, RJ10, RJ22 are what is on your phone handset or headset. Only works for some specific phones. Please see compatible phone models list in description below