Thank you Ardly, 6v6gt, Dan-n-J... Yesterday was not my first attempt to breach the fortress of SMS via GSM, and at the end of 5 hours, I was pretty much out of steam again, with a strong feeling that it Just Doesn't Work reliably, and if you get as far as that, you've built a house on sand...
Dave-n-J: Apologies of this sounds ungrateful, but for the sake of future readers: No, not a commercial product. Why would that matter? No, no internet access, or LoRad. And who would develop a 2G device today, with the 2G networks being shut down? Adding WiFi and LoRad and Bluetooth to a project that doesn't even work with the core medium that it is being built for makes no sense to me. DESIGNING it so that ADDING those things later WOULD be Good Practice... the sort of thing that a Good program has, whereas "One That Works" doesn't.
But you re-invigorated my quest by reassuring me that I have, sort of, grasped some of the basics. I do seem to have figured out some of the right questions!
And Ardly's "Try sending an SMS with your cellphone from the barn" was a wonderful example of how often a simple idea can be hard to arrive at. I WAS worried about the question of coverage... but (duh) hadn't realized that there was a simple path to a good answer! As I say... I've found the quest exhausting. (And I'm not one to "be beaten"!) ("Obsessive geek" is such a harsh label...)
(His point about designing the code in two parts is also excellent, and should be taken to heart by all. Two MAIN parts, and "parts-within-parts". Top-down design/ bottom-up development, and all that.)
My biggest worries now are...
a) "How long will support of 3G be there?" (No "answer" to that one, of course... but I point the danger out, for the next novice reading this thread. And, to the same, DON'T buy a 2G device. (And, by the way, some //devices// are region-specific.. and that's before you even get to the further restrictions imposed by your SIM card contract.)
b) Which 3G device should I try? At this time...
i) I lean towards the MKR1400...
(By the way: It does seem to require, for easily believed tech reasons, to REQUIRE the presence of a LiPo battery, even if you are continuously supplying a voltage to the system from other sources. Many sources will tell you "at least 1500 mAHr"... including one that looks to be an "official" Arduino datasheet. However, Arduino MKR GSM 1400, 16 Jul 2020 says 2500mAh Minimum)
ii) But the Adafruit Fona8 is attractive in other ways.
... and both are covered in "red flags" which say all this smoke can't JUST be coming from people who are trying to do too much with too little knowledge.
One of the biggest red flags: None of the suppliers show any signs of customers taking the product up. Comments/ Reviews pages are mostly empty. Stock levels are low. Etc. Nor do the suppliers seem to be putting much effort into the products. Very limited material or links to related products at their own site (batteries, antennae). And not many suppliers carrying it at all. Even the Mighty Amazon seems to yawn.
There ARE many ways to come to grief, even for my "simple" wants.
And I haven't even started on the SIM card issues! (Be sure to "go for" "MicroSIM" physical format... I think.)
Antenna... My best guess at this stage? The device MAY have an onboard antenna. It MAY be adequate. The board MAY have a "micro UFL connector" for connecting an external antenna. Antennae selection may be tied to the frequencies used by the 3G network you are trying to connect to. If so, GSM World Coverage Map- GSM Country List by frequency bands may be of help.
I have never experienced the trouble I've had in this quest... "Trouble" in the form of absent, conflicting, and contradictory "information", even when I apply "filters" to only consult reliable sources.
My bottom line: Embark on trying to use GSM with any microcontroller with a great deal reluctance and trepidation. You may well get greater pleasure from other projects. If only I could take my own advice!