Reddit reviews Headset Buddy PC Headset to Cisco Phone Adapter - Dual 3.5mm to RJ9/RJ10 VoIP Connection Cable (PC35-RJ9Cisco)
We found 2 Reddit comments about Headset Buddy PC Headset to Cisco Phone Adapter - Dual 3.5mm to RJ9/RJ10 VoIP Connection Cable (PC35-RJ9Cisco). Here are the top ones, ranked by their Reddit score.
Convert two 3.5mm plugs to single 4P4C RJ9/RJ10 Cisco headset plugWorks great with Cisco IP office phones with a RJ9 headset jackSave money by buying PC headsets to use with your office phone's headset jackDoes not work with RJ11 jack, which is wider than your 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 with mono and stereo PC headsets!
How about this? http://www.amazon.com/Headset-Buddy-Adapter-CISCO-Phone/dp/B003EALLHE/ref=sr_1_cc_3?s=aps&ie=UTF8&qid=1396466422&sr=1-3-catcorr&keywords=RJ+9+3.5mm
It's pricey, but seems to be exactly what you need.
Are you using Cisco phones at work? I got one of these for my setup and it works great.