I would be surprised if there's a free SMS gateway where users don't have to pay for it.
What I use is either SMS server tools or SMS provider in my country. This provider has an API, you can receive and send SMS messages but you have to pay for it, of course.
If you come across a free SMS service, I will be more than happy to know about it.
The good thing with SMS Server Tools is you can get a special flat fee for unlimited messages (if your mobile operator has this kind of special offer/plan). On the other hand, with SMS Server Tools you have to connect mobile phone or modem to your computer, it's not a tool for shared hosting where you can't access the server.