Ask HN: Are online free sms services illegal?

There dozens of sites for sending free online sms worldwide. Most of them are ugly websites with tons of ads, looks like they are built in 5 minutes. But the service works. I follow them, after sometime they shut the service down, something happens. I don't know why, and I'm not sure how do they do it.

In most of countries SMS is very expensive and there is no way to send SMS for free. I'm sure there are millions of people would use this kind of service. Would it be possible to build a paid service for sending SMS worldwide? Is there any legal way? I know that there are some API services like Twilio but they are too expensive. What do you think?