IOS – iPhone 5 Car Bluetooth Issue with Incoming Phone Calls

I have an iPhone 5 running iOS 10.2 and a 2015 Chevy Mailbu with Bluetooth capability for calls and streaming audio (non CarPlay).

The problem I have is when I am receiving calls (everything else works fine). When a call comes in and I answer it, regardless if I press the phone button on the steering wheel or the answer button on the phone, the iPhone immediately switches to its internal speaker I have to manually switch it back to the "Bluetooth headset" (car speakerphone) for the audio to work properly.

I have tested this in 3 different cars – the Chevy, an Infiniti and a truck with an aftermarket Kenwood radio. The problem is present in all three vehicles. I have also tested with a Windows Phone and and Android phone and both phones work with the cars properly so it's definitely a phone issue.

I have rebooted the iPhone, deleted the profile and re-paired with the radio and I have even reset the iPhone to factory default and re-paired it as if it was new. The problem persists no matter what I do.

Does anyone know what the issue is or what I have to do to solve it?

[Update]
Problem still persists after iOS 10.3, didn't resolve the issue.