Another option is to use your own "in-house" GSM modem, this can be as simple as a mobile phone with a data cable or something like this device (which I've used the older version of quite successfully) -
http://www.gprsmodems.co.uk/acatalog...odems_116.html My application was to use it to send text messages to mobile phones when there was a Nagios alert, the reason for using an "in-house" modem meant that there was no need for network connectivity to a third-party to send the text messages. Essential if one of the things you want to monitor is your internet connection!
Also, if you want to use this in an intranet then you may not want your intranet server to have external www access.
As for software, you can use this
http://smstools3.kekekasvi.com/index.php to drive most GSM modem type devices, it's fairly easy to use and a well formatted text file created in a folder will be sent as an SMS.
I would avoid USB "Broadband" modem devices as they look like a bit of a pain to get going as most are designed around being used on Windows and do a few tricks like switching from being a USB disk to being a USB modem through software. Although I'm sure Google might give you results for these devices if you're feeling brave.
Hope this helps.