I can’t send international SMS

Here's what I'd try:

Compose a new message and manually key in, in the address field, +1, area code and phone number without any spaces, hyphens et c. E.g. if the phone # is (202) 555-1212 enter it exactly as +12025551212 and enter some text in the text field and send. If it fails, once you get the Not Delivered indication, in the conversation in the Messages app, tap on the label up top and then tap on the info symbol and then tap on the contact name or # and see if the number with the RECENT label is exactly as you entered it. If it's not, delete every conversation in the Messages app that could possibly have that phone # or that contact in it in any way, shape or form and delete every contact in Contacts that has the phone # in any form. Why not if you're otherwise going to restore and set up as new anyway? Then try again.

If you're absolutely unable to send SMS to that # unquestionably formatted even via a new message in a new conversation then the only other thing I can think of to try is, of course, restore and set up as new.

Just curious:

• You can send SMS to some international numbers but not others including some but not other U.S. numbers?
• What happens, what is the indication, of such not being sent?
• Are you sure you're not getting false negatives? I.e. what appears to be unsent is, in fact, not showing as sent in the messaging detail you find when you log on to your wireless account and look at that?

You're welcome and thanks a million to you for your consideration. Gracious! 😀

Last gasp, grasping-at-straws if not outlandish ideas if not already tried:

• try iOS update 12.1.1 released December 5th
• try it with both cellular data as well as iMessage temporarily disabled
• restore a backup from the work iPhone to the Xs on a temporary basis and see what happens

SMS is a carrier feature. You should start by contacting your carrier. And yes, sometimes switching phones messes up things in the account.

You should contact the people who you can receive texts from but can't seem to send and see if they have you blocked or muted.

I'm going to have to agree with your carrier on this one. It sounds as if you've done all the steps I would have suggested. You can try restoring it from a back up first. It might work and you'd lose less data.

Best of luck.

I've meant most of my suggestions to be temporary fault isolation measures, by the way, rather than permanent solutions you would have to live with.

That's good to know! My compliments to you for the update.