One thing to keep in mind is that email is not synchronous, so you can get an email and just not reply for a while. Of course you can ignore phone calls, but it's not the same as ignoring an email. Additionally, phone numbers are useful if you don't have access to data, for some reason, or need to contact someone in an emergency. As a counter to your point about remembering phone numbers, I don't remember most, but I remember very important ones (like my father's).