Abstract: A simple and inexpensive circuit for evaluating the level and frequency ranges of call signals in a telecommunication system. For level evaluation the circuit comprises a pair of oppositely poled threshold value switches and an opto-coupler connected thereto. For frequency evaluation the circuit further comprises at least two series-arranged retriggerable monostable elements whose outputs are connected to a logic circuit. The dwell times of the monostable elements in response to an applied signal establish the frequency range within which a call signal can actuate the output of the logic circuit, thereby indicating whether the signal is within that range.