Part of it is an image thing
800 number says "we're a business" (even if that "we" is really "I"). I used to try to project that, but lately I've moved towards a more personal relationship with my clients. Using email now for more of my communications means that I'm more guarded about my phone number. I give my number only to my established clients, and it's a privilege. It says, "you can interrupt me if you really need to." That's not a privilege I grant to just anyone.