How do I make iPhone 13 mini default to phone audio instead of speaker for all calls?

Re: "iPhone 13 mini audio defaults to speaker on all calls in or out? How to default to phone?"

Try checking the settings on the iPhone by going to: Settings >Accessibility > Touch > Call Audio Routing >

See if Audio Routing is set to "Speaker" and if so, change that..

If needed: Then if you want to use speaker-phone :

While on a call you can always choose where to direct the Audio:

Apple support's online article: While on a call on iPhone - Apple Support

Eg: says: "Adjust the audio during a call

To change the volume, press the volume buttons on the side of iPhone.

Or swipe down on the call banner, then do any of the following:

• Mute: Tap the Mute button.
• Put the call on hold: Touch and hold the Mute button.
• Talk handsfree: Tap the Audio button, then choose an audio destination."

Thank you kindly for taking the time to reply, and to let us know! Cheers! :-)