It is defined as any carrier that provides services across multiple local access and transport areas (interLATA).
Telephone exchanges are identified in the United States by the three-digit area code (NPA) and the central office prefix, which is the first three digits of the local telephone number (NPA-NXX).
Different exchanges, or central offices, generally serve different geographic areas.
For voice traffic transfer, IXCs use Tandem switches and SS7 signaling or softswitches and VoIP protocols and error correction[citation needed] ITSPs can thereby connect between VoIP to POTS, computer to computer, computer to phone, and Internet Protocol devices to other phone services.
In popular usage, CACs are often referred to as dial-around codes, because they allow "dialing around" the PIC.