You need to integrate with some SMS provider.
You just call its URL from Java and pass the details as query string. That is it.
If you google SMS gateway providers in India, you will find a lot of them.
What kind of application is this and how many users are you talking about?
If you want to go bulk, then use SMS providers as already suggested.
If not, the easy ways would be one or more GSM modems where you insert regular SIM cards and use telephony API. Probably even easier is a couple of cheap android phones that forward your messages.
Okay for desktop apps or self hosted webapps with small user base. But neither would be possible if it's a web app that's hosted somewhere else.