I've seen the "Spotlight doesn't search mail messages" thread, but haven't seen anything on this. Spotlight doesn't appear to search for a phone number, even the leading part of a phone number, e.g. area code.
How? It hasn't up until now (I was hoping that Spotlight would solve this problem because the phone app won't do it), and I tested it there again. Typing "817," e.g., in the phone app's search field doesn't find anything.