>If mobile numbers in your country are in the 2________ range, how feasible is it to add millions of phone numbers to your contact list to find out the number of someone? I think this is nonsensical.

If you're a state actor probably pretty easy. Get a couple thousand rooted remote controllable android devices (which you probably already have for other projects) and have them automatically add 10k phones numbers each. Then have them join public telegraph lists and check for matches. Now you have gone through 10 million phone numbers. Run it in a loop 10 times and you have 100 million. Might take a few days to setup and run.

I don't see why this is infeasible in any way to do if you have a moderate budget (ie: state actor).

edit: And if your target is in your jurisdiction then you probably have a good mapping of names to phone numbers already.