how we can send sms using asp.net ?

You should first choose a good SMS gateway:

http://www.aspdotnetchamp.com/2013/11/learn-by-experience-sms-gateways.html

Then you choose the plan according your need. After you signup with them, they will provide some API to cosume. You should include into your code and sent the SMS.The below url give a detailed tutorial about how:

http://www.aspdotnetchamp.com/2015/08/how-to-send-sms-from-aspnet-web.html

Let me know if you have any other doubts.

follow the link

http://www.aspsnippets.com/Articles/How-to-send-free-SMS-from-ASPNet-application-to-Mobile.aspx

To send SMS, you need to use third party tool.

http://www.aspsnippets.com/Articles/Send-free-SMS-in-ASPNet-using-C-and-VBNet.aspx

http://www.c-sharpcorner.com/uploadfile/b8d90a/send-sms-using-c-sharp-net/

https://www.twilio.com/blog/2016/04/send-an-sms-message-with-c-in-30-seconds.html

Refer below url its help to you.

http://www.aspsnippets.com/Articles/How-to-send-free-SMS-from-ASPNet-application-to-Mobile.aspx