In other countries these setups are fairly illegal because it bypasses the international call tariffs that the typically state owned telco company would be entitled to. A local domestic call might cost $.01 per minute and an international call $.20. They call it "bypass fraud".
But in the US, I'm not so sure since things are already deregulated.
US doesn't really have bypass fraud as a category, no; there's no real pricing difference based on the source of a call. Inbound international calls don't have to pay extra termination costs vs domestic calls and outbound international calls aren't paying much more than the cost of a local call + whatever the foreign carrier charges for termination. If you were doing bypass fraud in another country for calls to/from the US, you don't need SIM farms in the US, because you could just get a SIP account.
These boxes would be used for pricing arbitrage where a mobile phone user can get 'unlimited' calling or messaging but a bulk messaging/calling customer would have to pay something per message or minute, or to avoid customer identification or restrictions on message that would happen with a bulk account.
Yet interestingly the USA could put you in jail for wire fraud for 20 years for doing so even if you never interacted with the USA or any US citizens or ever stepped foot on their territory.
Not necessarily. A correspondent bank is simply a bank where the recipient bank has an account in the specified currency. For domestic currencies used only in certain countries, that bank will almost always be in that country. For currencies used globally - not necessarily.
This was kind of my experience with reporting a bug to Google as well. Some years ago I managed to upload a SWF file to "google.com" which allowed me to do an XSS and access anyone's gmail, contacts, etc. I reported it and they just initially never responded and I had to constantly follow up. It was seemingly a simple bug to fix but it took them a couple months and they eventually only paid $500. Being able to exfiltrate data out of someone's gmail account always seemed high priority to me but I guess not lol.