[newscracker]

What happened in Hong Kong was that the authorities created Telegram accounts and added thousands of phone numbers to their contact lists. From that, they got to know which numbers are using Telegram and then were able to do some more tracing. This flaw exists in WhatsApp and Signal too, where anyone who has your number in their contacts list (though you may not have their number in your contacts) will know the moment you join those platforms and will be able to see you on it.

When this design flaw came to be known, Telegram released a newer version where the user has more control on who can know that they're on Telegram. With that change, even if you had someone's number in your contacts list, you wouldn't know if/when they join/are available on Telegram unless they choose to make themselves visible.