When connected to Ford Sync, iPhone Auto Mutes when Connected to Outgoing Calls

There isn't any setting on the iPhone that would cause it to go to Mute by default. You could go through all of the iPhone troubleshooting steps, but I still believe that Ford is going to have to deal with it. The only post that I could find and information on the internet was from back in 2018, and that was not resolved, but it was on a much earlier iOS version. So, if you were getting it to work before, it is going to be difficult to try and troubleshoot on the iPhone.

1st Step would be to restart the iPhone. Turn it off and back on again.

If not fixed, then force restart the iPhone. Force restart iPhone - Apple Support

Next, try unpairing and then re-pairing the iPhone via Bluetooth. On the iPhone go to Settings>Bluetooth, and remove the car from the paring of the iPhone, close Bluetooth. Go to the vehicle and remove the iPhone from the list of devices. Turn the car off. Turn the car back on again, go to Settings on the iPhone, then turn Bluetooth on. On the vehicle, open the pairing instructions for Ford Sync to pair the iPhone. Pair the iPhone to the Ford Sync. Test. Does it work?

I suggest that you contact Ford about issues with their Sync system.

You're welcome. sometimes we find devices don't play well with each other. The more we keep connecting to our vehicles, the more confused they get. Good luck.

You're welcome. Hope you get it all working.