For a start, just because something walks like a number and quacks like a number does not mean it is a number. Some things that look like numbers are just not. Phone numbers are a perfect example: despite being made from digits, they are not actually numbers. When have you ever added two phone numbers together? Or found the prime factors of a phone number? (Mine has eight, four distinct.) The rule of thumb should be: if you’re not going to do any math with it, don’t store it as a number.
