No sound for either party during phone calls, and Siri & Voice Memos don’t work, on iPhone 7, iOS 13.6.1.

My sweetheart’s iPhone 7, running iOS 13.6.1, started

having the following problem today:

During phone calls, no sound from the phone,

speakerphone button “greyed out,”

and other party (caller or receiver) can’t hear my sweetheart. 

This happens without any Bluetooth device connected or remembered in settings.

Siri will not respond to voice input.

Voice Memos app will not let anyone start recording a memo. 

You push the red button over and over and nothing happens.

Sweetheart still gets texts over AT&T network,

lots of them, announced by a loud horn.

I reset Network Settings and rebooted phone ... that

didn’t help.

I checked for software update ... software is up-to-date.

I tried WiFi calling ... that didn’t work; same soundless problem.

I tried Facebook Messenger calling ... that didn’t work; same soundless problem.

I paired my Koss Bluetooth headphones with the iPhone

and phone calls work ok through the headphones’ speakers and microphone. 

I want my headphones back.

Any idea what’s going on?  Any advice? 

Thank you in advance!