how to setup tasker for bt on call
Solution: 
Profile : Event > Phone Offhook 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Enter Task: BT On, WiFi Off, Mobile Data Off 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Exit Task: BT Off, WiFi On, Mobile Data On 

As long as your headset is paired and on, it should reconnect automatically.