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.
Point being, if "who is using signal" is a question you want answered, it's far more trivial than having to acquire actual devices. Your oppressive regime could go from zero to black bag list in an afternoon.
Dunno if this is patched by Telegram in any way now. However, I don't see why it would be difficult for a program to add numbers to the contact list incrementally. To my knowledge, computers so far were pretty good at incrementing numbers. And if the contact list length is limited, the question is just how many phone numbers a company can buy.
But you have no correlation between it and Telegram user. This bug is about this correlation.
Telegram has essentially agreed to tell you whether any phone number is correct, so you can just guess all the phone numbers. Never allow this unless the thing an adversary has to guess is both _completely random_ and from a _very large keyspace_ (128-bits is where you can start to feel safe). If you find you're cornered into doing this (e.g. typical email + password login) aggressively rate limit it, so the adversary has to work harder/ longer to take advantage and maybe they'll give up.
Phone numbers are neither random nor from a large key space, it's maybe 10^12 worldwide or something? Much too small.