a salt for a password is when we know the username and the salt so can combine the salt and provided password to compare against the generated hash value.
in this case all we know is the phone number, how does a salt help us look up a given phone number in a set of salted / hashes if we dont know what salt to combine it with?
You shard the number into groups with a salt for each, until you reach a number of combinations that is computable for FB size on a check request, but way too long pre-generate. You can combine it with some bloom like filter to increase efficiency.
a salt for a password is when we know the username and the salt so can combine the salt and provided password to compare against the generated hash value.
in this case all we know is the phone number, how does a salt help us look up a given phone number in a set of salted / hashes if we dont know what salt to combine it with?