We've recently added the ability to send SMS's in FinalBuilder using the ICQ network. Unfortunately it's quite limited because the ICQ network only supports a handful of networks.
Another way to send SMS's is via SMS gateways... there's literally hundreds of them and they are all different (ie. different pricing models, different way to send SMS's etc, etc.). The “Intellectual mollusc” on his FinalBuilder blog has just released an FB action to send SMS's via the Clickatell gateway. Worth checking out if you need to send SMS's via FB and your cell phone network is not supported by ICQ.