I presume you're talking about SMS messages.

What you need is a carrier that will do the sending over the network. That carrier will have an API. For a previous employer, I wrote an interface for $CARRIER - which wasn't any more difficult than sending an XML document over HTTP, using basic HTTP authentication, and a message digest + shared secret for authorization. LWP, some XML module and Digest::MD5 was needed.

It was pretty trivial to write, so I never bothered to cut throught $WORK[-1]'s red tape to get the non business-logic released on CPAN. The hardest part was figuring out what was missing from $CARRIER's spec, and which parts $CARRIER had forgotten to implement. ;-)

The most important document you need is your $CARRIER's spec.