Abstract: An apparatus for interconnecting a telephone line and a calculator having a printer and display means, wherein input detecting means are provided for recognizing dialing pulses and charge pulses on the telephone line, and output driving means are provided for acting upon the calculator only at points thereof corresponding to input keys and switches thereof which usually serve also to introduce values and give operating orders to the calculator, this output driving means being arranged first to control the printer for causing it to print telephone numbers in accordance with the number of dialing pulses detected by the input detecting means, then to control an addition process in the calculator for causing its display to indicate the charges successively incurred during a call, and finally to control the printer for causing it to print the total charges incurred during this call after the line has been restored.