An area code that covers the same geographical area as an existing area code. As a result, callers within the area must dial a full 10-digit number (NNX-NNX-xxxx) to reach any other number within the area.The concept of the overlay area code was developed as a means of avoiding forcing subscribers to change telephone numbers after an area code split. Such telephone number changes cause great disruption, result in lost business and otherwise cost businesses great amounts of money for advertising, re-printing of business cards and letterhead stationary, and so on. See also area code.