By the way, there's a centralized DND registry in India called "Commercial Communications Customer Preference Portal"[0]. You just need to send an SMS to enable/disable unsolicited commercial calls/SMS. It has been very effective so far.
It's a pain in the ass but I kept on reporting for a month after activating DND and the calls and messages dropped. I've even received calls from people begging me to take my complaints back or otherwise they'll loose their jobs. I agreed twice but holy hell those exact same companies spammed me again. I stopped being a good samaritan and just told them to sort it out with their companies since it's the companies fault. Now spam has reduced to a message or two a month.
From what I understand there are old mobile numbers list being passed around from before DND was available and that's why you need to start reporting the spam messages. Word quickly spreads between the companies to remove your number from the lists.