If you're going to cheat by using your contacts database, why not just ask your colleague for the original data? Even with hundreds of millions of observations

If you're going to cheat by using your contacts database, why not just ask your colleague for the original data? Even with hundreds of millions of observations, the space of possible hashing algorithms that map phone numbers onto those hash codes is infinite. Well, not infinite, but the space of functions that map phone numbers onto whatever domain the hash codes have is mind bogglingly large.