If you want to send/receive SMS using Modem then this link might be helpful. If you want to have large scale capabilities to send/receive SMS then you can contact SMS Resellers like Gateway4SMS. Search google for more such providers. There are plenty of them in India.
Besides these technical things, you need to be aware that it is illegal to send bulk SMS from your mobile phone in India. You need to have 5 digit special codes (like 57575) or 11 character (TD-IRCTC) name to send bulk SMS which will be provided to you by bulk SMS resellers.
If you are thinking to contact service providers like Airtel or Vodafone to connect to their SMSC (SMS center) then be aware that Airtel has pulled out of this business because it is ethically not so right. Your only options are Vodafone/Tata teleservices but those guys don't respond to you if you are small time player.