Abstract: Coin telephone set comprising a telephone station adapted to be connected to two wires of a telephone line from a telephone exchange, a call charge signal detecting circuit connected in parallel to the telephone station, an assembly of control and monitoring switching means operable by coins, respectively, logic circuits for processing the output signal from said call charge signal detecting and the output signals from the assembly of control and monitoring switching means, and a computing unit interconnected with the logic circuits for keeping up to date the account of the user of the telephone set from the moment the latter is connected to the desired telephone subscriber. The computing unit comprises a series of D-type flip-flops having each a clock input, a signal input, a resetting input, a setting input and an output, and a series of elementary adders equal in number to aid D-type flip-flops.