They definitely do not delete it entirely because sometimes a phone number is prevented from being added to more accounts for a few days or weeks, saying "This

They definitely do not delete it entirely because sometimes a phone number is prevented from being added to more accounts for a few days or weeks, saying "This phone number is already registered with another account" even if it's been already deleted from all accounts, though they may be just storing a hash of the number that's not linked to any user account.