Audicom Headset Adapter for 2.5mm Headset, RJ9/RJ10/RJ22 Headset Plug Adapter Converter for Avaya 1608 1616 9620 9630 9640 and 9650 and Altigen 805 Cortelco Fanvil C58 C58P Telephone IP Phones Convert 2.5mm headset 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. 2.5mm headset adapter. 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 2.5mm headset and some specific phones, do not work with 3.5mm headset. Please see compatible phone models list in description below.