Like email alerts, you have several options to getting this working.
Your options are:
Enabling an SMS service on your Nagios server
Using your own SMS gateway on another server
Using a commercial third party SMS gateway
There are several tools you can use to enable a server to send SMS's. Each tools has it's own strengths and weaknesses. The following list should help you decide what to use.
* Gnokii - Gnokii is very popular and can be used as a full-featured 2-way SMS gateway.
* SMSTools - An alternative to Gnokii, also popular and heavily documented.
* SMSClient - Calls a provider by modem and sends SMS's only.
If you'd like to build your own SMS gateway using Gnokii, I wrote a "SMS Gateway How To" a few years ago. You can read the original article here (http://www.chrisburgess.com.au/sms-gateway-how-to/), or read on for the simplified steps. I am assuming you are using the same server to run both Nagios and Gnokii.
Installing and Configuring Gnokii for SMS Alerts
make install clean
MORE STEPS FOR GNOKII COMING SOON