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. Why buy an expensive headset just for your Aastra office phone? Use the Headset Buddy Adapter and your existing 2.5mm phone headset with Aastra office phones that have a handset or headset jack. A big productivity enhancer for home businesses, call centers, sales people, or anyone who is on the phone all the time, but doesn"t want to buy two headsets, one for a 2.5mm phone, and one for your Aastra office phone. Specs: This amazing adaptor converts the standard phone headset 2.5mm plug to an Aastra RJ9, RJ10, or RJ22 handset plug, allowing you to use your favorite computer/laptop headset with your Aastra office phone. The adapter outputs binaural sound for stereo headsets, meaning you will hear sound out of both ears. The adapter cable is approximately five inches long.