I think you'll need to read up on the concepts of SMS protocols and gateways to figure out how to get started. There are built-in packages in Oracle 10g and 11g (such as UTL_HTTP) which allows you to communicate via web protocols and even to consume SOA web services. Perhaps if you set up a webserver which can send messages based on calls to a web api or a web form, you can leverage such a package.
If you're looking for something like "DBMS_SMS" or some other convenient PL/SQL code that will directly allow SMS messaging, I am highly doubtful that it exists except in custom implementations where the programmers have taken the time to figure out how to interface their messaging servers with their database servers. You may not be able to leverage the SMS app purely with PL/SQL. It may require a hand off to some other code written for the purpose of SMS messaging. Therein lies the challenge.
Hope that helps.
Rich Pascual