I don't think that they use a paid API to give the service. They somehow have a free access for sms sending. But most of them get banned after some time. Actua

I don't think that they use a paid API to give the service. They somehow have a free access for sms sending. But most of them get banned after some time. Actually if there would be a legal way, it can be very good business model. For example; 1$ monthly subscribtion for 1000 sms worldwide monthly. There some voip services, but they are too expensive.