If you have a Person's contact record, a contact could have N-number of phone numbers, i.e. by class of number such as work, home, cell, fax, and a highly normalized model would put all phone numbers in their own table. e.g. A fax number as a PhoneNumber with PhoneType=3.
What if application requires that contacts be retrieved with phone numbers? Those numbers in the contact record could in fact be a better solution.
Which one is the phone number to be used from N-number of phone numbers?
When storing phone numbers in multiple columns of a single row you experience different issues. What if a person doesn’t have a land line anymore? Do you put cell phone in the home phone number field…it isn’t home phone. When you want to print out a report with phone number, you need to write a query with a case statement or coalesce in order to find the first populated phone number.
What do you do when you need to record multiple phones of the same type? Personally, Person might have three cell phones used for different purposes. How do we know (if we need to) which type of connectivity that number represents?
if Person A and Person B have the same phone number, should they reference the same phone number record? If not, you still have duplication of data. But if so, you have a harder problem to solve when one person’s phone number changes, but not the other.
How to store a fax number ?
A fax number should be stored as a PhoneNumber with PhoneType=3 in a separate phone table?
All of the issues with having multiple phone numbers can be resolved as business rules. Implement those business rules in stored procedures, functions, etc.If you want to restrict your objects to having three phone numbers such as work, home, and cell, then implement that design in your business objects and business rules.
If your database doesn’t force you into that business rule, then when you get a new requirement to allow for tracking another number, your database schema is not impacted; only your business layer.
Regarding the uniqueness of a phone number and Person A and Person B have the same phone number(it being shared by multiple people); normalize to the point of having a many to many relationship between different entities and phone numbers with which they may be associated.
Regarding how to store a fax number, a fax number in a FaxNumber field is far easier to understand than a PhoneNumber with PhoneType=3 in a separate phone table, so Greater understandability of data.