iPhone won’t connect to Toyota vehicles?

...and Hyundai.

My husband has a Toyota Prius and his iPhone will not connect via Bluetooth. I also have a Toyota vehicle and his iPhone also will not connect to my car. In the beginning of our relationship I thought it was his phone (iPhone 4) so I gave him mine when I upgraded (mine worked perfectly with my car) so when he began using my iPhone 6 i thought the problem would be fixed. It was not. From then we have upgraded many times and his phone still somehow will not connect. He’s had different cars and the Bluetooth does not work with any of them.

The problem: when he attempts to connect his phone he has to do it immediately as the car is turning on and sometimes it will not connect at all. When it refuses to connect it shows “connection successful” then the screen reboots over and over and over until he turns off his Bluetooth on his phone and disconnects from the vehicle.

lm at wits end and he is considering switching to Samsung (which would be inconvenient since all of our devices are Apple) so I’m hoping to get some answers or point me in the right direction.