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).
You can get a shortcode in three days - $500 for a random 5-digit, $1,000 for a vanity 5 or 6-digit. The biggest gating item is the approval process with the carriers. That will take 6-8 weeks.
If you're getting a prototype working, textmarks might be the way to go. You'll have to prefix your SMS command set with a keyword. Longer term you want your customers/members to identify with your own shortcode. Imagine them getting messages from two services on 41411
Right. The approval process with carriers is what makes getting a short code so time consuming. What was it like working with the Aggregators? I've heard mixed stories.
mBlox was good in getting us feedback and suggesting fixes during the process. You spend most of the time waiting for the carriers to respond to applications and testing.
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).