anization of the PSTN. Prior to the breakup of the Bell System, all telephones were connected to the local exchange office. The local exchange office was responsible for monitoring each line to detect when a customer went off-hook, then to connect the appropriate resources to de- tect any dialed digits, and then to signal the customer that dialing could commence by providing dial tone. The digits collected would then be used to determine how to con- nect the call. The local exchange office had the additional responsibility of monitoring the length of the call and creating a record of the call start time, length, and called number [called an automatic message accounting (AMA) record] for use in rendering a bill for the call. This local exchange office occupied level 5 in a hierarchy of switches in the network and alternately was called a class 5 office. Each class 5 office was identifiable by a three-digit number called the office code. 3 The lines within the office were identifiable by a four-digit number. This combination of three and four digits was the seven-digit telephone number that customers would dial (when within the area code) to reach an- other telephone. The three-digit office code was cleverly designed SO that the digits 1 and 0 never ap- peared in the second position of the code. When a class 5 office saw the three-digit code with neither a 0 nor 1 in the second position, it knew to expect only 7 digits. Once the dig- its had been collected, the switch would use its internal parameter database, called trans- lations, to determine what to do with the call. If the office code belonged to this class 5 office, then it would terminate the call locally. If the office code did not belong to this class 5 office, then it would examine its translations to determine what trunk to route the call out over. If the call were to another class 5 office to which it was connected directly, then it would select an interconnecting trunk. If there was no direct connection available or all direct connections were busy, th