It's not free. You pay for every single message. (The more you buy, the cheaper they are - from 5.7 cents to 2 cents a message) However, they do supply a fully integrated solution via their API which provides 2 way communication between your application and theirs including world-wide coverage and a service level agreement.
We charge our users for sending messages over the 50 free credits they get when they sign up, adding about 2.5 cents to the cost of the message meaning we make a very slight profit per message sent.
You get what you pay for. Just depends how much you're willing to pay...
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