Text not delivered

My texts to my friends who don't have an iPhone don't get delivered. This happens whether I have SMS on or off. My carrier supports unlimited texts. These people used to get texts from me no problem, but starting this summer, whenever I send a text to these friends the message has the red exclamation point in a circle and "Not delivered" in red under them. They haven't blocked me and have done nothing on their end to stop my texts coming in. I can send and receive just fine in texts with them on my other phone. The phone with a problem is an iPhone X and the one without the problem is iPhone XR. Of course I have tried starting and restarting the phone.