It looks like that's how they're doing it since they have the caveat of "may receive messages from txt@textbelt.com" As for the open source comment -- they're p

It looks like that's how they're doing it since they have the caveat of "may receive messages from txt@textbelt.com" As for the open source comment -- they're probably running a carrier lookup on the numbers via another API on the back-end, so that dependency on another service doesn't make it as straightforward as just open sourcing it.