TRAI (telecom industry regulator in India) has asked all operators to stop ISD calling on prepaid numbers until especially asked for by customers.
TRAI says that it has been receiving complaints that people are getting missed calls (wangiri calls) and calls relating to winning prizes or lotteries from international numbers. These are often premium numbers charging high tariff, prompting consumers to call back such numbers. By responding to such calls/SMSs consumers have to pay unintended charges.