2017 Legacy hands-free phone transfer setting

Dear Sirs,

I have a 2017 Legacy that I use with my iPhone SE while driving to receive/make phone calls via Bluetooth.
When I initially set things up, it worked flawlessly, piping sound through the car's sound system when I would receive/make a phone call.
I don't recall making a specific change, but recently, every time a call comes it rings properly, but when I accept the call, the car switches to phone audio, and to use it, I have to press the 'Transfer to phone' to make it use the car audio.
I have not been able to change this behavior back to how it used to be, and I cannot find a setting, either in the phone, or the car.
How do I make the default audio to be the car instead of the phone?

Any help would be appreciated.

Sincerely,
Duane