The NCPR aka Do Not Disturb registry is a national database containing a list of the mobile numbers of all the subscribers who have registered their preferences (by dialing or sending an SMS to 1909 or by registering using the TRAI DND mobile application) regarding receipt of unsolicited commercial communications. As per TRAI guidelines, companies are prohibited from making Unsolicited Commercial Communication to subscribers who have registered themselves in the NCPR. In short, 'Promotional Calls or SMSs' cannot be made or sent to the customers who have registered themselves in the NCPR.
If a person is NOT registered in the NCPR, you can choose to make a Transactional or a Promotional call to them.
For eg: If Mr. Rajni who is registered in the NCPR calls your Exotel number on October 1st, his number will be whitelisted till March 31st (6 months). This means you will be able to make transactional calls to him during this period. But from April 1st, you will not be able to call him unless he calls your number during that period.
But in the above example, if Mr. Rajni is not registered in the NCPR, you will be able to make a transactional call even after April 1st.