alt Hacker News> Carriers would probably hate this and might not be willing to sign roaming agreements with such a company.
This is THE problem with your idea. Congress would have to pass a law forcing them to do it, or they won't.
You'd probably have more luck physically keeping someone's SIM card, keeping it installed in a phone, and watching for new texts. Perhaps you could make a box that simulates 10 phones at once.
> congress would have to pass a law forcing them to do it
Well, I'm not so sure about that. SS7 redirection attacks exist, so clearly shenanigans like these are very hard to stop for carriers. The question here is whether such "attacks" are legal if performed with the consent of the customer, but against the wishes of their carrier.
One could also do some "legal optimization" here, and ally themselves with a major carrier outside the US. There are plenty of those, and all of them have access to the networks (SS7 and IPX) on which roaming happens.