Also true caller adds security when you're from an under represented or a minority groups. I especially know women who use Truecaller to make sure they know who is calling / texting them from new numbers.
Having multiple SIMs in India will go away, or at least become something only the rich have, it's only a matter of time. This is because having that second SIM is no longer cost-free.
Multiple SIMs in India were a side-effect of it being near-free to have a prepaid SIM to receive calls. This was subsidized by high calling charges (calling from one part of India to another was expensive) and even higher data prices.
Then Jio entered the market with a ground-up 4G network and said, this is BS. Calls and texts are free, with no reasonable limits. We'll only charge you for data, and we'll provide 1.5GB a day or more -- starting at INR 150 a month paid 3 months at a time. The party ended for a lot of operators at that point as they hemorrhaged customers. Most operators simply folded or merged until now there are only 3 private players and one non-serious state player.
Driven by pressure from Jio, the other two private players (Airtel and Vi, aka Vodafone) have decided to amp up monthly charges for pre-paid phones. Essentially, if you're not spending at least INR 120 a month they don't want you. India has number portability so it's not like you're held hostage or anything.
INR 120 sounds super low but it's not for a lot of Indians who earn salaries closer to India's median per-capita income, and also it's an extremely limited level of service, for actual use you need to pay more -- typically around INR 250 monthly for 1.5GB data per day (remember, most Indians don't have wired Internet so 1.5GB data per day is not excessive). Jio and its competitors have hiked prices substantially, of course.
The last refuge for people on limited budgets is the state-run telco, BSNL, but the government will sell it -- it's only a matter of time. Expect prices to spike again, then.
Anyway, multiple SIMs in India are increasingly less viable for ordinary people. It'll take a while for behaviour to change, but it'll change.
I wonder what the percentiles is for number of networks in populous and large countries. Perhaps 3 networks is the number of maximum economically viable long term networks due to the enormously high cost of installing all the fiber and cell station infrastructure across the country.
They're broadly compatible from a handset perspective - sometimes there's better coverage for one or the other in some area or newer tech but for most day-to-day use you can get a SIM from any carrier and expect to be able to use it any (non-network-locked) device.
If I am not mistaken some shell company bought the spectrum then Jio bought the shell company.
That's everywhere, not just India.