Phone numbers are unique to every customer, so it should be used as a unique identifier and so should be a required field. We get very confused when we see 2 "John Smith" and not know who is who. In fact, we need phone numbers more than we need last names, as last name requirements tend to slow us down when making a new customer profile (especially if the last names are foreign and very long), if we just had their first name and their phone numbers, that should be enough to create a client profile.