Why can’t I make phone calls to anyone other than another iPhone user

Cellular phone calls are a carrier feature. Start by calling your carrier.

If the other people you can't call are on T-Mobile they are experiencing a widespread outage.

I'd managed to forget about that.

If you're an AT&T or Verizon customer, apparently, you can't call TMobile customers.

Verizon and AT&T had minor outages earlier today but those are gone. Can't text or call T-Mobile people. I blame it on their taking in Sprint.

I'm prepared to blame most anything on Sprint. They've always been pretty abysmal.