They are used to route and bill long-distance calls in the public switched telephone network (PSTN). 

• Purpose of CICs: CICs are primarily used to identify the long-distance carrier selected by the customer for routing and billing long-distance calls. They ensure that calls are correctly routed through the network of the chosen carrier and that billing is accurately processed.

• Format: A CIC is typically a four-digit code. This code is dialed before the long-distance number to specify the carrier for that particular call, especially in systems where multiple long-distance carriers are available.

• Billing and Administration: Carriers use CICs for administrative and billing purposes. The code helps in accurately tracking the usage of network resources and in generating bills for customers.

• Evolving Use: With the evolution of telecommunications technology and the increasing use of mobile and VoIP services, the role and usage of CICs have changed. However, they remain an integral part of the traditional long-distance calling infrastructure.

Carrier Identification Codes play a critical role in the management, routing, and billing of long-distance calls, providing a mechanism for consumer choice and competition among service providers in the telecommunications industry.