Telemarketers have been issued 10-digit telephone numbers starting with 140 (140XXXXXXX) by the Department of Telecommunications on TRAI’s request.

Customers (landline and mobile) who do not want to receive commercial communications can dial or SMS to 1909 (toll free) and register in either of the two categories:

Fully Blocked Category- stoppage of all commercial Calls/SMS
Partially Blocked Category- stoppage of all commercial Calls/SMS except SMS from one of the opted preferences

For registering option using SMS, for ‘fully blocked category’, write “START 0” and send it to 1909. For ‘partially blocked category’, send SMS ‘START’ with one or multiple options from the list of seven categories.

There are at present 7 preferences to choose from- Banking/Insurance/Financial Products/Credit Cards-1, Real Estate-2, Education-3, Health-4, Consumer goods and automobiles-5, Communication/Broadcasting/Entertainment/IT-6, Tourism-7.

For example: To receive messages relating to only Health products, then send SMS “START 4” to 1909. Similarly, for receiving messages relating to Real Estate and Education, send SMS “START 2,3” to 1909.

source: http://en.wikipedia.org/wiki/Telephone_numbers_in_India

and http://www.nccptrai.gov.in/nccpregistry/