In my previous organization, i'd written a sms gateway in vb 6.0 that was using a third party tool (air2web) to send SMS. The gateway had schedulers running which brought relevant details from various sources(oracle, sql server, excel files) at regular intervals defined in a 'Rules' table to an smsFetch table and a timer used to run which used to read and parse data from this smsFetch table and used to send the sms. The vendor had given us a URL with which contained userid/pin, mobileno and smstext as parameters of the URL and the timer used to invoke for each row of data.... Charges were INR 0.18 per sms for national sms and Rs 3 for international sms.

There are other vendors who provide dll's which you can incorporate (if you're planning to write a short program for it).