I see that nobody replied to your post here. I thought I posted an answer elsewhere, but I'm too lazy to go find it. In any case, this is a known/reported firmware bug, planned to be fixed "soon".
In the mean time, you can use the "old fashioned" method... plug your headset's base unit into the OBi phone's handset jack instead of the dedicated headset jack. Plug the cord from the handset into the headset base's handset port. Use the manual switch on the headset base to control which device is used, and answer the call by picking up the handset and setting it off hook, instead of using the headset button on the OBi phone.