Customer Proprietary Network Information (CPNI) is defined by the FCC as

a) information that relates to the quantity, technical configuration, type, destination, location, and amount of use of a telecommunications service subscribed to by any customer of a telecommunications carrier, and that is made available to the carrier by the customer solely by virtue of the carrier-customer relationship;

and

b) information contained in the bills pertaining to telephone exchange service or telephone toll service received by a customer of a carrier.

Practically speaking, CPNI includes information such as the phone numbers called; the frequency, duration, and timing of telephone calls; and any telecommunications services purchased by the customer, such as call waiting. CPNI, therefore, may include highly-sensitive personal information.

For further information on the FCC's definition of CPNI, please visit www.fcc.gov CC Docket No. 96-115.