How Usernames Protect Phone Number Privacy:
Enables Anonymous Contact:
Once you have a username, others can find and contact you by searching for that username in Telegram.
They do not need to know or use your phone number to initiate a conversation with you. This is the primary mechanism for decoupling your communication identity from your personal phone number.
Hides Your Phone Number by Default (with telegram number database proper settings):
If you've set your phone number privacy to "Nobody" under Settings > Privacy and Security > Phone Number > Who can see my phone number?, your phone number will never be visible on your profile to anyone, regardless of whether they found you via your username or not.
This means if someone finds you by searching for @YourUsername and clicks on your profile, they will not see your phone number displayed.
Controls Discoverability by Number:
Even if you communicate via username, someone might still have your phone number from outside Telegram. Telegram provides another setting: "Who can find me by my number?" under Settings > Privacy and Security > Phone Number.
If you set this to "Nobody", people who only have your phone number (and not your username) will not be able to find your Telegram profile by searching for your number.
If you set it to "My Contacts", only people who have your phone number in their device's address book (and use Telegram) will be able to find your profile by searching for your number.