[nfriedly]

I think it's much less likely to have collisions because the input space is so restricted (just valid phone numbers).

[jsjohnst]

SHA-1 has approximately Random distribution, so it doesn't matter what the input is really in practice.

[saurik]

The point of the person you are responding to essentially comes down to pidgeon hole principle (the best principle), and absolutely applies to a cryptographically random function, and is agnostic to the input distribution: it is a relative measure of information/entropy between two sets.

[jsjohnst]

I agree with what you said, but I don't agree that's what the person implied by their post.