Send too many SMS messages, fast way, but different messages

I'm working on a project which will send a message each time a user logged in. I know that there's so many ways to do that, like using GSM modem and maybe web service.

But the problem is that I need to send like 10 messages per second, because in 10 minute there will be like 1500 user logged in. Furthermore, Each message contain different message text. What's the best and fastest way to send the messages.

P.s.: I don't care about which technique is good for this, either hardware (maybe ARDUINO and GSM) or just software (like a web service).