To curb the growing menace and effectively regulate unsolicited commercial calls and messages, Telecom Regulatory Authority of India (TRAI) has notified "The Telecom Commercial Communication Customer Preference Regulations, 2010". The regulations comes into effect from 27th Sep. 2011.

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,
Consumer goods and automobiles-5,

Examples - 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.

On successful registration, customer will receive an SMS confirming exercised options and a Unique Registration Number within 24 hrs. The registration will be effective within 7 days of placing the request with the service provider. Subscriber can check the status of their registration in NDNC Registry by clicking on "DND Registration

Customers can also de-register their request for NDNC registry by dialing 1909 or SMS to 1909 STOP

Customers not registered with NCPR can also identify the commercial calls because they will all come from numbers starting with '140'. This is a series specially earmarked for telemarketers.

Also, in order to control the possibility of SMS packages (those allow a very large number of SMS per day) to be used by the telemarketers for sending large number of SMSs from a normal telephone number, a limit of 100 SMS per day per SIM has been laid down under these regulations except on blackout days.

Further, all commercial calls or SMS except the transactional SMSs will be sent to the customers only between 9 AM and 9 PM.
For more details please check Telecom Commercial Communications Customer Preference Portal

Source : How to fully or partially block telemarketing calls / SMS