I just posted about a headset on an IP phone. Then I found the solution, so I deleted my post. Then I thought I should post what I found, in case it helps anyone else, and in case you were reading the post that got deleted.
Anyway, my user complained that his new wireless headset worked, but he had to scroll a page or two on his IP phone to get to the headset key.
I found the solution on ntac: Headset on SL2100 IP phone
It seems the phone has to have this function enabled in the phone.