Most GSM Phones / Modems, even the early ones, have a AT Command set, so what you would need is a data cable for your phone, RS232 I would presume, and then you need a MAX232 or equivalent chip, and (test with a PC and hyperterminal / terraterm pro) issue an ATDXXXXXXX where XXXXXXX is your phone number.
Google this gsm circuit at command pic The second link on google, click that The 4th post in the forum has a circuit. I would have posted the link to it, but it is another forum. A simple google search will generally get you close. The modem/computer will be your phone. There are more links in the search as well.