I have the exact same problem and have had it since iOS 8.0. iOS 8.0.2 and 8.1 had no effect on the issue. I've never had problems pairing, receiving incoming calls through the car, or playing music. The problem is only with outgoing calls. Under iOS 7.x, when you made an outgoing call, the radio would go silent as soon as the outgoing call started ringing and you would hear the ringing through the car's speakers. Now under iOS 8.x, outgoing calls will not go through the car until either the other side picks up or the call goes to voicemail. Only then you are able to manually flip the call to the car's bluetooth. The button has no effect until then. The call will never automatically flip to the car.
Under iOS 7.x:
1. Dial wife.
2. Have conversation.
Under iOS 8.x:
1. Turn down radio.
2. Dial wife.
3. Set call audio to speaker phone.
4. Wait for wife to answer.
5. Tell her to hang on.
6. Route call through the car.
7. Have conversation.
Not only is this new arrangement a pain and a step backwards in terms of functionality, it's also a lot more dangerous when driving. I was hoping iOS 8.1 would address this issue, and maybe it did for some cars, but not for mine.
iPhone 6, iOS 8.1
2009 Mazda 6 Grand Touring