There are alternatives as well...
It is a fairly simple task, if you simply need an indication of an event (without transferring any info). Have two GSM modems: one on the site and the other at your control station. As discussed in the above, on alarm conditions at the remote site, dial the number for your control station (command ATDxxx\r, where xxx is the telephone number. The other modem will detect the call, start to pulse RI (ring indicator) of serial port and send "RING" to serial port.
If you want to be sure that the call comes from the specific number, you can either set "waiting for call", or restrict the allowed incomming callers, or set the modem to tell you who is calling (using the appropriate CLI-related command on your modem, and getting a string like +CRING voice, yyyy).
Practically speaking, working with GSM, gives you an ability to establish different types of calls (e.g. voice, data and fax) and you could use this as another indicator of the type of alarm you are making.
Sincerely
Albert Lysko