You can configure Kannel (an open source SMS gateway) for use with your GSM modem/phone. However, as others have pointed out, this does not scale.

On top of the SMS gateway you'd also need to rent a number or lease a short code in order to receive messages. Short codes can take up to 90 days to become active and cost ~$1000 USD/month.

There are companies that allow you to use a shared dedicated short code and have an API to deliver mobile content - some will even do this for free (http://www.textmarks.com).