First and foremost thing is that By customs baggage rules only one mobile device is allowed free of tax/ duty for PERSONAL use only
Later u can register on DRS of DIRBS free of cost.
Currently no phone has been locked despite warnings of PTA
Now no more airport registration only online. And as stated above only for personal use one mobile device is allowed free of tax / duty
So by law if only you have came from abroad yourself and brought a mobile with you only then you can register it free of tax /duty for personal use only