For BSNL,Vodafone,Aircel,Airtel,Idea,Reliance,Tata Indicom Subscribers:
sms START DND on 1909 (toll free).
And in case you want to de-register yourself from the NDNC registry sms STOP DND to 1909 (toll free) or call 1909 (toll tree).

For Virgin Mobile Subscriber Visit:
http://www.virginmobile.in/how_do_not_disturb.php