Headset Buddy 2.5mm Headset to RJ9/RJ10 Phone Adapter for CISCO phones ONLY (PH25-RJ9Cisco) Convert 2.5mm plug to CISCO phone single 4P4C RJ9/RJ10 headset plug Save money by buying standard phone headsets to use with your Cisco phone 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. Works only with CISCO and ATIVA phones with headset jacks