Check the output of your rsync in the script, if it returns anything other than 0 then you'll want to send an alert.
As for sending an SMS alert you are best to subscribe to a commercial service, many of these allow you to send an e-mail and have that forwarded to your mobile. You'd be very lucky to find a "free" service to do this unless your mobile provider provides an e-mail to SMS gateway.
Alternatively you could attach an SMS modem or mobile phone to your server and use something like
SMS Server Tools to drive the modem. Beware of "stick" USB/Broadband modem devices though as these can be a pain to get working.