iMessage switching from SMS to iMessages

Thanks. I should have provided a few more details:

We both had 50%+ power,

both phones have imessage turned on

both phones are signed into out apple ID,'s

and he did not move location between green and blue messages.

not that I can recall. Thank you for the suggestion. Not sure that is solution, since the issue has never happened in the 3 years prior, and just started in the 2 days I have had this phone, but will keep eye on it to see how often it reoccurs. Thanks

have you seen that other message before: "multiple phone numbers assigned to your apple ID"

Even if the phone did not move and the blue/green messages were less than 5 min apart?

No that wasnt it, either. That message never came up before upgrading the Xr. He's had his own number and Id for a year or so

Yes, same phone number on the new phone. Simply upgraded device from the 6s