Also, if you know the operator, many times they have email gateways that you can use to send SMS messages. It's very useful since if the person responds to your email message with their mobile, you'll receive their response in your email. This makes it more like real SMS messages and less like you're just "paging" them via a webpage.
For example here's my email address (Telefonica's Movistar):
686361410@correo.movistar.net. Feel free to test it out and I'll try to respond to you... My wife will think I'm suddenly very popular. (Keep in mind the +6 to +9 hour differences in timezone with the U.S.!!!)
I'm pretty sure that Amena and Vodafone have the same thing, maybe someone can find their info...
-Russ