Its not a exact sms script of any kind its just a simple edited version of scripts on the net...

If you ever have made a email contact form before then making a sms form will be easier..

If you want to include all sms companys like t-mobile/verizon/sprint...
Then your form needs to include a menu select (which company) then the phone number box (which number its gonna be sent to) then the company sms gateway email...

t-mobile = @tmomail.net (sms and mms)
verizon = @vtext.com

Find a list here..
http://en.wikipedia.org/wiki/List_of_SMS_gateways

its better if you create your own so you can customize it better..
All your doing is sending a sms through the gateway like to send one to 8888888888 at t-mobile then you can send a email to 8888888888@tmomail.net (thats the us email)