Why can’t I make and receive calls with my iPhone 6s in 2023?

There is no technical reason why a properly-functioning and carrier-provisioned 6s can’t place and receive calls.

If you are having difficulties doing so, contact your carrier.

If you can't make or receive calls on your iPhone - Apple Support

Why can’t I make and receive calls with my iPhone 6s?

You can.....if the phone is operating correctly and is authorized by your carrier.