If it was only between your phone and tablet, I would suggest
Tablet Talk . Can be connected through bluetooth or wifi. Incoming call notification let's you pick up as well as select what source (phone or connected bluetooth device) to use.
Also awesome for responding to text messages as well through your tab letting you leave your phone in your bag/pocket.
Other little features like auto starting/ending the app when you toggle bluetooth on either device.
As for phone and phone, you're definitely going to have to look hard for a bluetooth headpiece. I've seen the bluetooth visor speakers allow 2 simultaneous connections but not headphones personally.