Ironically, perhaps, the car everyone must be complaining about in this thread introduced call transfer controls that avoids just this. Through an OTA update.

Yes, it's Tesla.

If you're already on a call when the car connects to the phone, you must press an on screen button to transfer the call to the car audio.

(Bloody brilliant, if you ask me; and also something that couldn't be introduced to existing cars post release that were buttons only. I was quite anti-screen before I got mine, but now that I've gotten firmly used to it - and seeing enhancements every few months, I'm quite happy. I would like more generic mappable buttons on the steering wheel, but it's not a deal breaker).