and searched this site for an answer without finding anything. I did some experimenting and there are a couple of things to check:
1. Make sure the jack is pushed ALL THE WAY IN. It may indeed by so in your case but I think in mine it wasn't;
2. It appears the button is sensitive to how long it's pushed in. A brief push answers/hangs up-a longer push does something else but I'm not sure what.
Just my $.02.
(and I DON'T have a BT headset on the way. Stuck with this one).