If you enter someones phone number into the search box on Facebook, the site can perform a reverse look-up and tell you who the phone number belongs to. The social network itself seems to consider this a deliberate feature. Facebook user in question was shocked and surprised that she had been able to find her profile simply by entering her mobile phone number. She confirmed that her privacy settings were correctly locked down to such an extent that her phone number should only be accessible to her friends.”]
Source: https://grahamcluley.com/phone-number-privacy-facebook/