PURPOSE: To offer a privacy call circuit with simple circuit constitution by utilizing that the frequency of a voice signal passing through a storage circuit in response to the ratio of input and output clock frequencies in inputting or outputting the voice signal to/from the storage circuit while being sampled according to the clock.

CONSTITUTION: A clock frequency at input/output is equal at all times by Equation III and a voice signal is reproduced without frequency distortion, since M+N clocks in total are received until a sound signal is outputted from a register 24 at a terminal B after the voice signal transmitted from a terminal A and received at the B is sampled in a register 3, in selecting a clock period J in a clock frequency ft from a clock generating circuit 4 to the relation of Equations I , II to bit numbers M, K, N, L of analog shift registers 3, 12, 24 and 31 and further in expressing ft as Equations IV, V. Since the clock frequency at input/output is not equal, however, other than Equation IV when the signal is transmitted from the terminal A, the frequency of the voice signal is changed in the ratio of f(ti+M)/f(ti) of the clock frequencies.