Sorry no clue on headset, hopefully someone else can chip in on that.
On my X10 large you double press the middle home button to get to your home screen, from there you can do other functions as required. To retrieve the call screen just drag down your notification bar at the top of the screen and tap on the current call. Or you can simply end it by pressing the power button. This function doesn't work on the mini's tho.
The menu button gives you options on various call functions, merge, add etc. One of those is bluetooth so that might be where you need to look for the headset.
