The headset jack appears to fit 2.5mm headsets, but I can't get any to work. You can hear but not be heard. Is there a specific adapter or headset model that I need?
We have tested quite a few accessory headsets for the 7921s. The compatibles are the (2.5 mm, 3 band - 4 conductor) models from Plantronics and Jabra.
Cisco Unified Wireless IP Phone 7921G data sheet
Hope this helps! :)