With a couple GPUs you could find out if there is a collision on phone numbers with truncated hashes pretty quickly. If the phone numbers are normalized, except

With a couple GPUs you could find out if there is a collision on phone numbers with truncated hashes pretty quickly. If the phone numbers are normalized, except for a few edge cases, the space is twelve digits which can be naively brute forced. A trillion SHA-1 hashes isn't that hard to do these days.