What was said above - most/all phones will only pair with one device at a time.
I've got one of these - http://www.amazon.co.uk/Belkin-Car-Audio-Connect-AUX-Bluetooth/dp/B004FNYQZ8/ref=sr_1_1?s=electronics&ie=UTF8&qid=1379258137&sr=1-1&keywords=aircast - which does calls and audio, it's wired in via the aux.
Call quality is on a par with the built in, possibly a little better I think as mic is in a better place (hard to tell - it's the people the other end who get to hear what I sound like!). To be honest, I rarely use it for calls as almost always got the roof down!