Hey there cattenam,
If the volume is high enough being emitted from both your Bluetooth headset and car stereo, the behavior you described could occur. This is due to the volume being high enough for their voice to be 're-heard' by the mic in your car/headset.
With the above in mind, we wanted to help rule out some possible causes, along with learning more about what's happening:
- Does this same issue occur if you make a call while your iPhone is not paired/connected to anything at all, and does this occur with all contacts?
- If there was a time where this issue wasn't occurring with those same two devices, when did this start?
- Since you have a compatible iPhone model, the latest version is iOS 16.4.1(a). You can verify this within Settings > General > About. In case you needed it, this link shows how to first back up, then Update your iPhone or iPad.
Cheers!