Ref: Send free sms in ASP.Net using C# and VB.Net
1. Register in Site2SMS and get Username and Password.
2. Then Register at Mashape and get the API key.
3. Download the DLL of ASPSnippets SMS API DLL
4. Add reference to your project and use the following code
Namespace
using ASPSnippets.SmsAPI;
Code
protected void Page_Load(object sender, EventArgs e)
{
SMS.APIType = SMSGateway.Site2SMS;
SMS.MashapeKey = "<Mashape Key>";
SMS.Username = "<Site2SMS Username>";
SMS.Password = "<Site2SMS Password>";
SMS.SendSms("<Recepient Number>", "This is a test message");
}