Hi Mamta,
One of the best ways to hire in bulk in such numbers has been networking and employee referral. By networking, I mean not just networking with your tribe, but also networking with your employees and the employees that you have hired. They would be the best source for bulk hiring as they would refer their tribe. If there is a referral incentive attached, however minimal it is, it always adds value and will actually speed up the process. Trust me, if you are thinking about bulk hiring in the long run, a good PR exercise with the old employees and the employees that you have hired during your time and a referral incentive attached to every referred employee will work wonders.
I have been part of a team that did close to about 150+ hirings in just a month, and that too for frontline sales (that's even tougher than sourcing for call center crowd!!) All the best in your endeavor.
Regards,
Darshana