My Galaxy S3 is no longer sending texts.  The problem started quite suddenly yesterday morning (11/18/14) for no discernible reason. Nothing changed about the phone between Sunday when it worked and Monday when it didn't.

I'm using the default messaging app that comes with the S3. As far as I can tell, it seems to be happening to every text I send, regardless of what kind of phone is on the receiving end. Tested it on people both with droids and with iphones. 

What happens is I send the text and I get the little "sending" icon, which on an S3 is a little spinning cirlce thing.  That goes on for awhile and then I get a message informing me that the text failed to send.  One time of the several times I've tried, I got the error message, but the person on the receiving end still received the text.

The only thing that changed is that I was roaming with voice/text only, overseas (Japan) for 2 weeks.  My texting worked fine while in Japan. However it has not worked since I returned.  Nothing about the phone in terms of apps or software or hardware has changed between when it worked and now.