iPhone 12Pro bluetooth to vehicle issue

Hi .. since up grading from an XR to the 12 pro, I have had issues with the connection to my car stereo for hands-free calling. They both appear to connect via BT but more often than not, when a call comes in or is made, the music on the stereo is muted automatically as per normal, but there audio for the call only comes through the phone and "speaker" has to be selected even though the head unit is shown as the audio source. Some calls don't mute the audio at all and are missed. I have removed each device from and re paired them, but this has made no change, BT symbol appears on the head unit to show they're connected. There were no issues at all with the XR, and both were running 14.4

Cheers

Guy