"... But what are some reasons a person blocks their phone number? ..."
cos they dont want the person being called to know who is calling or from where.

if you get an "unknown caller" call, just ignore it, period.
dont attempt to figure out why they're doing it - you'll just waste ur time.
it could be spam callers, scammers, or even guys attempting to hookup w/ you.

as said above, if it's important, they'll leave a voice msg or txt you.