Let me try and answer your questions.
Do you need to receive SMS? If you need to receive SMS, you will need to host your own GSM device or modem so that people can send you SMS.
If not, you can just use internet SMS gateways like clickatell to do the work, and post to them by HTTP, XML or email. The cost is about 6-8 cents per SMS. There are cheaper services, but not always reliable. If you need to host your own GSM device, you can use software like
Kannel: Open Source WAP and SMS Gateway (GPL Open Source) or
http://visualtron.com/enterprise/sms-gateway.
Regards,
SMS Gateway@Expert
