You can send SMS via your phone or via the SMS center (the box at the operator's network used to forward the sms's and save them in case the phone is switeched off...)
TO send trough the sms Center, u need to make a deal with the operator, or witha 3.rd party that has such a deal..
www.clickatell.com has deals with operators worldwide, and provide many APIs to help you send SMS from diffrent apps..