No need for any 3rd Party SMS Services like Twilio, just your WordPress site and This WP Smart SMS Plugin
Guaranteed to work in the U.S. only.
This may work in your country please go to our test page here: http://timkendall.7sms.org
If you do not see your carrier listed contact me so I can add it for you.
Here is my complete list of Cellular Carriers that support this Free Service.