Amikor bepötyögsz egy számot (USA-ban 7 jegyű) és távbeszélést indíthatsz azon a közvetítő távbeszélő szolgáltatón keresztül, akiknél előfizetésed van.

http://computer.yourdictionary.com/cac

CAC Telecom Definition

1. Carrier Access Code. In the United States, a seven-digit code (101 XXXX) that a caller can use to dial around the presubscribed interexchange carrier (IXC) to reach another IXC.The first three digits (101) of the CAC signal the network of the caller's intent. The last four digits (XXXX) of the CAC are the Carrier Identification Code (CIC), which is used for call routing purposes. See also CIC, dial around, and IXC.
2. Carrier Access Charge. In the United States, a set of access charges billed by the local exchange carrier (LEC) to the interexchange carrier (IXC) to compensate the LEC for the costs associated with connecting interexchange long distance calls over LEC local loop facilities. First, the CAC includes flatrate, recurring charges that apply for tandem exchange termination. Second, the Carrier Common Line Charge (CCLC) is a minutes-of-use charge that applies to each call connected to the IXC. See also access charges, IXC, and LEC.

IXC. Interexchange Carrier = közvetítő távbeszélő szolgáltató.

igazából "hozzáférés a hordozó közeghez", dehát ezt nem lehet így elmondani