I have this problem with my Kenwood receiver KVT-512 with Bluetooth module KCA-BT200. My AT&T iPhone 4S has worked fine for 3 years, my iPhone 4 before that, and my iPhone 3G before then. The iOS 8 update is a problem. I called Kenwood and they said I was the first one to report a problem with iOS 8 to them. They will not be updating this 6 year old Bluetooth module. So, it's on Apple to fix this.
I noticed it this morning when I got the first call since the update. My own number was shown on the Kenwood screen as the incoming call, and the answer button was dimmed so I couldn't press it. When I pulled the phone from my pocket to answer it, the head unit reverted back to playing my music. The only way I could get the call audio to play through the speakers was to go through the Bluetooth A2DP player.
When I re-paired both devices, I saw a behavior that others have described here. Immediately on pairing, the incoming call screen takes over my head unit and first shows my number, then the last number that called me, even though no call is in progress. The caller name has two words, one is Interrupt, I forget the other right now. The call screen never lets go and I can't do anything else on my stereo system. When I placed a call from my phone and selected the BT as the source, the BT mic did work and the sound did come from the speakers. But, it never showed the correct caller and didn't let go of the screen when the call ended.
I also tested this with someone else's Verizon iPhone 5 with iOS 8 that had never been paired with my BT previously. It worked perfectly fine, as my phone used to before the update. I don't know what's going on here. I'm trying to get an iPhone 6 and I wonder if that will work either.