I don't use a BT headset that often, but do use BT handsfree in the car, but not with 3CX. Just tried it on the 3CX phone Ver 3.0, (on my Samsung Rugby Pro) and find that there is no selection (in the audio section of the 3CX phone) for using a BT device, and, can confirm, it does not work with a registered headset. I'm assuming that there is still an issue that has yet to be overcome in Android.
There is now an update to Ver 3.0, which I just installed,will try later in the day to see if there is any difference.