I worked on Dinstar Gateway, vicidial and Asterisk PBX Voice over IP. For sending limited SMS GSM gateway is enough. Where as for bulk I had other one to suggest but the number which we will sending is from USA number there is no need to pay for the number. And we can change any number at any time. The message will directly can send to the Indian mobile numbers. For this we required a Gajim Software. I will completely install and show you the process.