Abstract: A computer based telephone accounting system collects data on calls made from a key system or PBX and automatically generates accounting reports which include the costs of the calls. A minimum amount of memory is used in calculating the costs by grouping combinations of area codes and exchange numbers into geographic bands and precomputing the distance from a particular area code and exchange number to all the remaining combinations of area codes and exchange numbers in the other bands. The combination of area codes and exchange numbers are stored in an N.times.M memory matrix where N equals the area code numbers and M equals the exchange numbers. A plurality of memory stored, multi-bit data words defining an algorithm for computing the cost of a call to a particular band are individually selected using an address defined by the position of the bit in the memory matrix which corresponds to the particular area code/exchange number being called.