Hi, I'll be glad to help you with this task. I worked for several years for a mobile aggregator and I know my way with all the SMS sending technologies and options.
So basically, what you need to do is to connect a standard GSM phone to the computer and we'll send to it AT commands to send SMS. First, through HyperTerminal, then on the C program we open a serial connection and send the same commands. In theory any phone with a valid SIM and active account will do.
It's not that complicated but I'm bidding to finish in 2 weeks because I have other commitments and to give us time to do tests and etc etc.
Later on I can help you with other options to send high volume messages, precisely what I did for several years.
Thanks and have a great week!
Sergio