If you want to send a free one then this one is for you.
http://www.aspsnippets.com/Articles/How-to-send-free-SMS-from-ASPNet-application-to-Mobile.aspx[
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And if you want to deliver paid sms then register yourself first to some sms portal and then you will get an url like this one.
http://000.000.000.000/API/WebSMS/Http/v1.0a/index.php?username=arkadeepde&password=******&sender=Arka&to=$mobile&message=$text&reqid=1&format={json|text}&route_id=7
Now you have to run the code with your values like
string url = "http://000.000.000.000/API/WebSMS/Http/v1.0a/index.php?username=arkadeepde&password=******&sender=Arka&to=" + txtMobile.Text.Trim() + "&message=" + message + "&reqid=1&format={json|text}&route_id=7";
WebClient client = new WebClient();
string rtn = client.DownloadString(url);
in
rtn get the json valu. from there you can show whether your msg has delivered or not.