Auto Pairing of iPhone with Toyota Prius
(Our Toyota RAV4 is not at home right now, otherwise I would go look in the manual to see what I did).

I know for our Toyota RAV4, that I was able to automatically pair my wife's iPhone with the car so that each time the car is turned on (and the iPhone is in range of the car), the bluetooth system takes over and lets us answer the phone through the car. 

My suggestion is to look in your owner's manual and look up how to affect a default pairing. When my RAV4 comes home in a few hours, I can look up the steps we used.