How to transfer phone number from old iPhone (5c) to new iPhone (12 mini)? Merely switching SIM card doesn't work.

Trivial but frustrating problem.

Had an old iPhone 5c. Unlocked. Account with MetroPCS.

Got a new iPhone 12 mini. Unlocked. Initiated the 12 mini, works perfectly, software updated to latest iOS.

Everything about the 12 mini functions correctly EXCEPT that if I pop the SIM card out of the 5c, and put it into the 12 mini, even though the phone accepts the card and recognizes the phone number, for some reason I can't make calls or send texts from that number. Try to make a call, it says "Call failed." Try to send a text, it says "Not delivered."

In Settings>Phone>My Number it lists the correct phone number, apparently successfully reading it from the SIM card.

In Phone>Contacts>[my account name] it says "Phone" and lists the correct phone number.

What is going wrong? I suspect that there is some very simple step or setting that I ought to know about but have not done, that is preventing the new phone from adopting and identifying with the phone number on the SIM card.

Any help or suggestions would be greatly appreciated.