If you deleted the contact from both Telegram and your phone's address book, the first step is to re-add them to your phone's address book.
Go to your phone's native "Contacts" app.
Create a new contact entry.
Enter their correct phone number (including country code) and their name.
Save the contact.
If you have "Sync Contacts" enabled in Telegram (Settings > Privacy and Security > Contacts > Sync Contacts), then after you've re-added the contact to your phone's address book (Step 3), Telegram should automatically detect them again during its next sync cycle.
They should then reappear in your Telegram contact list. This might take a few minutes.
Method 3: Manually Add Contact in Telegram (if "Sync Contacts" is disabled or preferred)
If you prefer not to sync contacts, or if Method 2 doesn't work promptly:
Open Telegram.
Tap the "+" (plus) icon or the contact icon (often telegram number database a person silhouette) to add a new contact. The exact icon varies slightly between Android and iOS.
Select "New Contact".
Enter their first name, last name (optional), and their exact phone number (including country code).
Tap "Create" or "Done" (depending on your OS).
Telegram will then check if that number is associated with a Telegram account. If it is, the contact will be added to your Telegram contact list.
Important Considerations.