Not true. DND works great in India. You can also report a spam call or an SMS and your mobile network provider has to provide a resolution. They usually do it by banning that abusing number.
DND is available both on Android and iOS.
DND is available both on Android and iOS.
So unless the phone company starts banning phone companies that don't enforce correct caller id, it won't do anything.
But it evidently does do something, since now it's confirmed in these threads that at least 4 (possibly) more countries do have these lists, and they do work.