Telegram's Privacy Settings that Affect Discoverability by Phone Number:
The ability to find someone by their phone number hinges on their settings, specifically:
"Who can see my phone number?" (Settings > Privacy and Security > Phone Number)
"Nobody": If the user has set this, their phone number will never be visible on their profile to anyone. This is the strongest privacy setting.
"My Contacts": Their phone number is visible only to telegram number database people who are in their phone's contact list and who also have the user saved as a contact on Telegram.
"Everybody": Their phone number is visible to anyone who views their profile.
"Who can find me by my number?" (Settings > Privacy and Security > Phone Number)
This setting becomes available if the user chooses "Nobody" for "Who can see my phone number?".
"Everybody": Even if their number isn't displayed on their profile, anyone who searches for their phone number can find their Telegram profile.
"My Contacts": Only users who have the person's phone number saved in their device's contacts will be able to find their Telegram profile by searching for their number.
In summary: If a user has set both "Who can see my phone number?" to "Nobody" and "Who can find me by my number?" to "My Contacts" (and you are NOT in their contacts), or even "Nobody" for both (if they've taken extra steps like not syncing contacts or using a Fragment number), you likely won't be able to find them by just their phone number alone.
How to Try to Find Someone by Phone Number (If Allowed).