You should have one outside phone line for every five employees, depending on the type of business you're in and your call volume. Still not sure if you need more phone lines? Get a "busy study" from your local phone company that will track the number of times a caller receives a busy signal on your telephone. They may charge you a small fee for this service, but it'll be worth it.

Excerpted from Start Your Own Business: The Only Start-Up Guide You'll Ever Need