Related: If you see notifications about people you don't know, please see this card.
When new users from your contact list join Telegram, a notification about this appears in the chat list.
The New Contacts option in Settings > Notifications and Sounds currently determines if you receive a push notification about the
'Contact joined Telegram' message.
Mobile carriers routinely recycle phone numbers that are no longer in use. Telegram has no ways of knowing when a number from your contacts changes hands – and will display it under the name you have saved.