Anytime I think about these issues and this model I always wonder:
Can you get a cellular connection over a wire?
That is, instead of having 500 little radios connecting to one or two nearby towers, can you negotiate a direct connection to the tower and use the entire cellular stack except for the PHY ?
This is pretty much what we have been asking every supplier (telcos etc) over the past 2 years. The answer is always no. And if it is a "Maybe, I think so" it turns into a "no" weeks or months later when have finished digging through the corporate hierarchy.
The only solution that seems to work is old school SIM card hosting in a SIM bank. In some narrow cases, e.g. sender is in the country and receiver is in the same country, you might have pretty good (95%+) reliability of receiving critical SMS (A2P traffic), but still far away from what you'd call reliable.
There exists FOSS that could do this too (start with "osmocombb").
But the real problem here isn't technical, it's a business/legal issue: the carriers and their regulators are trying to minimize (or at least, reduce) the ability for bad actors to operate large numbers of "cell phones" at minimal cost/complexity.
So everything that could be done (technically) to make this work is, in practice, prevented by those business/legal considerations.
You won't need the air interface - hypothetically just an appropriately rooted femtocell, carrier HSS/HLR/MME that can authn/authz you, and Asterisk server that is secure. Or a flooded Nokia Flexi on a rack shelf, I mean, they look cool, don't they...
