You'll know if a call is coming from an inside extension or an outside line. You have a report to finish and the phone keeps ringing. Activate this feature and calls will not be put through to your phone. The system also accepts relays to allow the door to be opened remotely. This refers to an intercom phone number in an office, not to be confused with a phone line.
Several extensions can share the same phone line.
Another way to think of extensions is to compare them to electrical extension cords. Extension cords enable more than one appliance to use the same electrical outlet. These can be completely different from one phone to the next. This way you don't have to learn new extension numbers. The system conforms to you, not the other way around.