Account Codes, otherwise known as Long distance access codes are a great way to keep track of your long distance usage. Each user would be assigned a 2,3,4,5, or 6 digit number that they would keep as a "pin".  When making a long distance call, after dialing the number you would be prompted by a tone to enter your pin. Only a correct pin would allow the call to be connected.

Then when you recieve your invoice, each pin is marked by the name assigned to it, and a list of all of the calls connected by that person. Detailing a summary by totals and by individual usage.