My iPhone 5 (I had same problem with iPhone 4 and iPhone 4S) is connected to my car's bluetooth system for phone use. (It's a 2006 Acura and only supports bluetooth for phone.)
When I make calls from the phone, the audio defaults to the phone itelf rather than the bluetooth connection, making me have to tap the selection for my car's bluetooth audio to switch it.
When I receive calls, they initially come to the phone via bluetooth but after answering the call, it switches off bluetooth and directly to the phone about half the time.
I've tried this with Android devices and I don't have the problem.