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Ok, switched my SIM with my woman's SIM for a short period. Looking at the Messages app, there was some nonsense added in our conversation about a message due to time out next month sometime. This seemed very odd so I clicked on it and was offered the chance to download it ?!?!?!
It downloaded twice, once on my side of the conversation, once on my woman's side of the conversation (she says she's never seen it). Looks like something I probably tried to send her, but the inexplicable bit is that it looks a medium sized text (293 characters apparently, nowhere near the 480 that it should cope with) all just text, yet at the side it CLAIMS to be MMS not SMS. How the heck can it choose to be MMS when it's been given no indication whatsoever that it should EVER have anything to do with MMS ?
Anyway, I can't be sure the issue has been solved, I can only wait and see if the warning no longer appears. And whether switching the SIMs has changed anything. (I've not noticed anything so far.)
What gets to me is how utterly useless Samsung Support was though. All they ever did was a combination of nothing, and the occasional suggestion of me asking my service supplier to sort the data connection, before closing the ticket. No real solution at all.
Anyway fingers crossed all is well, but I'm not holding my breath.