I saw this from user Timult on Nov 10, 2020. It makes the most sense. We’ll see.
this has to do with settling the phone number.
1. From the Home screen, tap Settings > Cellular (iPhone)
****I had to turn off Cellular Data first ***
2. Tap Network Selection.
3. Slide the Automatic slider to the off position to start scanning for networks.
4. Select desired carrier (T-Mobile)
*** Turn Cellular Data back on ****
- Restart phone
- Settings > Messages > Turn on Send as SMS
- Click Sent & Receive
- Make sure the Number and Apple ID are listed and correct
- Remove the check on your phone number by tapping it. And select Remove.
- Now that it’s unchecked, tap to apply it again. This reconnects your phone.
- Done.
See Timult full post for with pics