I think the main selling point of the BT feature is to do
ground calls, to save using the radio/battery. But, in Europe, most of the useful numbers are confidential and anyway I doubt the controller would take the call - even for a departure clearance call.
I have done such calls where I know the number, to check if e.g. they had received a slot.
But given one is on the ground, one doesn't need a BT headset; one can just use the phone
Unless the engine is running.
The other use is airborne, with a BT equipped satphone, like the Thuraya model(s). The call quality is really crap though.