Hi,
Basically you require a SMS Gateway through which you have to send your message, which would be a paid service to which you have to subscribe. My suggestion would be to create a common interface for mail, text messages etc which would contain methods like "SendMessage","ConstructMessage" etc and implement those. Usually your application would require to consume a webservice provided by the gateway service, but I believe there are some other methods in practice as well. The following links might be of help to you -
http://forums.asp.net/t/57735.aspx
http://www.gotocode.com/art.asp?art_id=66 - contains the basic code structure
You can download this free component and check it out (not sure about licensing terms, so read their terms if you want to use it commercially) - http://www.intellisoftware.co.uk/sms-gateway/dotnet-component/