Like you said, it is very effective in lodging the complaint, but that is as it seems, as effective as it will ever get. The first of the "investigations" that were done was for a "weight loss" sms spammer using an MTNL Delhi ID, the reply they gave "No action taken as this number does not show any telemarketing activity". Case closed. The rest simply have chosen not to respond.
I think these so called "investigations" are just a sham, they'll probably only check if its a registered telemarketer or not and then action it.
From what I've observed, they are not that effective at blocking spam from shortcode based senders. I've successfully gotten every single random 10 digit mobile number disconnected who somehow got it into their heads that I'm in dire need of a massage.
It was after wondering how all of a sudden this spamming started, it dawned upon me, I had registered my Flipkart account just a few weeks ago with it. Hence, Flipkart seems to have directly or indirectly given/sold my number out.
Anyways, back to using localised blocking
If you buy from any third party seller on these marketplaces, it will most likely end up in some list for sure. A lot of different folks end up having access to your details - the seller, the shipper etc. etc. And it's not as if all these companies have excellent data security practices, bound to be leaked or sold, just a matter of time.