This solution isn't free, but it's pretty close. I'm using twilio.com to send and receive SMS messages in other applications. Twilio has a well documented interface, and you can get a unique phone number for $1.00 a month plus $0.01 per SMS messages sent or received. I tried setting up an ISY network resource based on the SMS sending message at http://www.twilio.com/docs/api/rest/sending-sms and when I ran a test, it popped right up on my phone with my twilio "from" phone number, just as expected. Of course, I also got the error "TCP client read response failed [Net Module Rule: 4]" from the ISY which I'm uncertain how to interpret, although that didn't interfere with my ability to use the resource in an ISY test program to send messages.
They have free demo accounts available for testing, so if you're interested, you may want to give it a try.
Here is screen shot of the network resource setup, with a bit of blurring over my account details:
Chris