robotmay who also replied to you is correct for what I do, I have multiple prepaid US SIMs with PAYGO rate plans. It's trivial to get many of these and register them in bulk to get "real" mobile numbers.
(This is why I laugh and laugh at people who insist that VoIP numbers are more "fraudulent" because of whatever. Right now, breaking no rules whatsoever, I can get a hundred mobile numbers that any service will accept, and they'll go for a year without needing any more money. Stop using phone numbers as identity verifiers, people. All you're doing is making it harder for the people who aren't technically savvy and accomplishing nothing to prevent actual fraud.)
It's been a while since I looked, but the cheapest US paygo plans are still at least $3/mo (eg H2O wireless), and probably double that if you want more convenient billing. Have you found something better, or are you deriving that much utility from scaling to many devices or what? I'd think if the goal is signing up a bunch of accounts, one would be content with a single plan or maybe two, and just periodically swapping their phone numbers.
It seems like it would be easy enough to change your setup to hide your actual location by locating the gateway elsewhere and backhauling with its own IP connection.
BTW are there any standard interoperable formats for transmitting/presenting/archiving text messages, akin to Maildir for email or SIP for voice? If not, maybe Maildir is the right answer.
How so? None of the fraud prevention APIs I've seen tell you if a given number is connected to a base station, only whether it's theoretically GSM, landline, VoIP, or something else. With the ability to tunnel LTE over an unstructured Internet connection ("wifi calling,") I don't have to be within range of a single US-based base station to "appear" like I am in the US to the mobile network.
And everyone tells me that only accepting "real" mobile phone numbers means they're just piggybacking on the identity checks that mobile carriers do. Which, if true, I'd like to introduce you to Constable George Crabtree and his nineteen perfectly valid mobile numbers, all of them with his name in the CNAM field.
All this does is prevent someone who's not tech savvy and who might be trying to save a bit of cash by using a Republic Wireless or TextNow or some other "free calls and texts!" service from fully participating in this new app-based reality we've constructed. Someone who actually wants to commit fraud will step right over these dumb speed humps and do whatever they like.
I'm not saying don't do fraud detection, I'm saying don't do fraud detection that is so screamingly trivial to bypass.
In the UK you can get a SIM for free from most networks as pay-as-you-go (i.e. you add credit to it instead of getting a monthly bill). With most of those you can happily receive messages on it without ever spending a penny.
(This is why I laugh and laugh at people who insist that VoIP numbers are more "fraudulent" because of whatever. Right now, breaking no rules whatsoever, I can get a hundred mobile numbers that any service will accept, and they'll go for a year without needing any more money. Stop using phone numbers as identity verifiers, people. All you're doing is making it harder for the people who aren't technically savvy and accomplishing nothing to prevent actual fraud.)