I would recommend taking a look at Textlocal, India & UK's No.1 bulk SMS provider. Textlocal has unparalleled delivery rates which is of the upmost importance while sending a SMS campaign. read more
Not cheapest, but definitely the best sms gateway provider in India. Currently serving clients like Deccan Herald, IIT Kanpur, Vijaya bank, A.P Social welfare ministry, Prime Focus Technologies and many more. read more