Car bluetooth issue - Audio output switches to "iphone" when answering

Im having issues with an iphone and hearing calls via bluetooth.

Im able to successfully connect my phone to bluetooth and can hear the phone ring through bluetooth, but when I go to answer the call - the audio output instantly switches to iphone audio (have to hold ear up to phone to hear it).

When I go to select the output option as the vehicle (on the iphone call screen), it switches right back to "iphone". I am able to select "speaker" option.

Note:

• Other iphones work fine - just mine has the issue
• Ive tested it in another vehicle, and I get the exact same issue (other family members dont)

Thefore I suspect the issue is with my device.