Last night, I put the SIM from my phone into the iPhone we will giving my daughter for Christmas to see if it worked. I used my wife's Apple ID to set the phone up.

Today, all texts to me from iPhone users are going to my wife and not to me. I suspect my phone # is now associated with my wife's Apple ID and Apple has a messaging service that bypasses how texts would normally be sent and sends them to her instead.

How do I fix this?

Edited to add:

I've deleted the new phone from her Apple account.

I've "deregistered" my phone number from iMessage, but it said my number wasn't registered with iMessage in the first place.

Any iPhone you log into you an Apple ID will associate that phone number with that Apple ID, and since your SIM was in the phone, that's how it happened.

I have two iPhones (personal and work) and iMessage works on both, quite seamlessly, along with an iMac, MacBook, and iPad.

Deleting the phone from her Apple ID account should take care of it, and be sure to log back in on your own phone with your SIM.

Go to "settings"
Click on "Messages"
you will see "send & receive"
Click on it and remove your phone number and email address from that phone.

Do the same on her phone and remove her number and email address from yours

I've had success following cndrdk's method. I think Lefty's suggestion may be more permanent. I've noticed after updates sometime I have to go back and "re-bolck" numbers.

Thank you for the suggestions. I had done a reset and erase on the new iPhone before I put it back in the box to be wrapped.

I couldn't find any setting on my wife's iPhone that would switch it from using iMessage to SMS when texting my number. I did notice in the iMessage settings that it had her old Apple ID listed; one that was an email address from an ISP we stopped using two years ago. I switched that to the iCloud Apple ID I set up for her when we switched ISPs. I then tried sending a text to my phone and it said undeliverable, switched from iMessage to SMS by itself, and the next text I tried worked. I also received texts from other iPhone users today, so it seems to be fixed.