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Many carriers allow you to send emails to {phone_number}@carrier.com which become text messages for free.

See here: https://github.com/typpo/textbelt/blob/master/lib/carriers.j...

OK. But I am curious how it maps a phone number to a carrier. My understanding is that carriers do not publish the list of their customer phone numbers.

It seems like textbelt does not know this information (or at least when you sign up or send a text message). Does it try in sequence to send the text to all carriers one by one until one succeeds? If so, I'd imagine textbelt would get blacklisted pretty quickly.

It would be interesting to see how the mapping is done.

That's an interesting question, but I think that Textbelt is intended to be used in a scenario where you ask the user to select their carrier in addition to their phone number.

Thanks for posting this. I asked the question when I was on my phone but it made more sense when I got back to my desk :) I wrongly assumed Textbelt was using a GSM modem to deliver messages as opposed to the email-SMS gateway approach.

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