I didn't have a phone capable of WiFI calling on AT&T: any practical use of the new free plan seems like it would be needed.
I found an AT&T Prepaid Radiant Core on clearance at Target for $15 , and It's supposed to have WiFI calling.
I don't have one of the new SIMs to test yet, though.
If anyone is interested, here's a link to see if your local Target might have this in stock, and current price.:
https://brickseek.com/target-inventory-checker/?sku=080-02-0186