Every device that makes or receives calls will be attributed an extension number. An extension will use a line when making or receiving a call.
In most cases, an extension can directly call another extension by simply dialling the desired extension number; this type of call is referred to as an internal call.
Extension numbers are shared by the organization or business and can be in different offices, locations or even countries, making long-distance calls between branches an internal call, thus saving you money.
A public phone number can be assigned to one or multiple extensions.