I was wrong. I didn't actually have to do anything. All my own efforts were in vain but much to my surprise the problem was fixed. By none other than my service provider.
Don't know why, except that as soon as it started to work, I received an SMS from them. Hence, maybe they have the power. Wouldn't surprise me.
Anyway, good luck with it all.
I have a T-Mobile S8+ and it does the same thing with outgoing text messages. (and was purchased directly from T-Mobile).
Sometimes outgoing SMS works fine and then, all of a sudden, it won't work. I can still get incoming messages though. I switch from WiFi to cell...messages still fail. Switch back to WiFi...still fail.
What I found was that if I switch back and forth but then copy the text, delete the original and then create a new message and paste the text...then they will all of a sudden send instantly again.
Seems like an issue especially right after receiving an SMS while I am sending an SMS. Like the radio gets confused receiving while trying to send.
Just thought I would share my experience, and frustration, with a T-Mobile purchased (not 'unlocked', not Verizon) phone.
Tmobile Galaxy S8 (not unlocked). I had this problem. Factory Data Reset, did not allow any updates or wifi during the process. Didn't allow Samsung updates. SMS worked fine for about a month. I recently noticed the problem starting again. And sure enough, Messages had updated to the lastest version...
Is there any way to roll it back?
The Answer to some of The questions.
Restart your setting.
The factory has a Bug in which it makes the Messaging Locked.
Restarted and it works fine.
Setting/Connections/More Connection Settings/Restart Network Settings
And it will restart it to Normal
I am having the same issue, the only work around I have found is to use the Android Messages app. I would like to use the Bixby messages feature but unfortunately the messages app will not cooperate. Please fix this issue soon.
Same issue here with T-Mobile G8. Already tried factory reset, sim replacement, and ultimately phone replacement. I started using textra and it solved the problem. I just hate the ads. I'm going to try android messages now. Some app or widget has to be interfering with the native messaging app. I use Bixby and Google Assistant a lot. I have a suspicioun that this is the culprit. My problems didn't start until i started using those regularly. Frustrating. I hope it gets resolved soon as if prefer to use the Samsung app.
Also running into the same issue of not being able to send SMS with a generic 950U on Boom wireless. Everything else works. Tried textra and android messages with no luck.
Sending SMS did work for a few days but for some reason the some card detected new settings and then SMS stopped working.