Ask HN: Are online free sms services illegal?

Though I am unable to answer your question of whether a legal and profitable SMS system can be built, I'd like to point out a hidden answer in your statement — "after sometime they shut the service down, something happens. I don't know why"

Most of these "ugly websites" are fly by night operators. They are spammers looking for in-use phone numbers to build a database. The database then could be used to send bulk spam SMSes or could be sold to tele-marketers for a profit. So essentially these websites never make any profit from actually sending the SMS but instead scam the users to share their phone numbers.

Now you know why!

I was thinking about this a few months ago, but I couldn't find a way to make it affordable.

Showing banner ads doesn't seem to be enough, since the cost to send one SMS is greater than the profit from one banner. Perhaps that's why those sites had so many banners?

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.

I'm using this service http://smssheep.com