Some kind of service outage has hit Apple authentication servers for iMessage. On Saturday iMessages stopped going through for me, and when I checked iMessage, I was logged out. After several conversations with Apple, I finally got elevated to someone who said "Yeah, it isn't you, it is us. Apple engineers are trying to fix it now. Hopefully it will be fixed soon."
One of the things I noticed is that iMessage works on my laptop and iPad, but none of those is using my phone # as the send-from. Instead they are using my iCloud address and the phone number has disappeared from preferences on those devices. So beware that if iMessage is working, or appears to be working, the send-from address may be an email tied to your iCloud account and not your phone number.
I can still text with my phone using SMS by selecting SMS and shutting off iMessage (which otherwise says "Waiting for activation.") But that doesn't work with all recipients for some reason, because they often have iPhones and my phone number is recognized as an iMessage number, so the person sending the text has to know to switch the message to SMS. It doesn't switch automatically, probably because it still arrives on my laptop and iPad.
Headache.
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