My OnePlus 5T Bluetooth for telephone in car not working.

[Android Central Question]

Recently, the telephone call/receive function for the Bluetooth in my car has stopped working on my phone and in the car. When I select the telephone symbol to make a call the screen on the car is greyed out and it says "service unavailable". I have rebooted my phone countless times, re-paired my mobile to the car and it still does not work. Yet, my partner has exactly the same phone that I do and his works! Can anyone help me please and let me know if I have screwed up something in the settings? Both phones have recently had system updates and the phone was working this morning in the car and this afternoon it doesn't? Funny thing is that the media still works and I can still play music - it's just the making and receiving calls that no longer works I really need it to be working as I drive a fair bit. Any help is appreciated and thanks in advance