frequency sinusoidal signal which gives rise to the ringing signals and an enabling circuit controlled by the exchange components for enabling the supplying of the ringing signals to the line and for activating a circuit for detecting a direct current on the line the circuit for detecting a direct current on the line supplies a signal to a circuit for controlling the detection of an off the hook condition and which supplies an inhibiting signal for the enabling circuit and check only after a predetermined time whether the circuit for detecting a direct current on the line is still supplying a signal and for generating a signal informing the exchange components that the set has been off the hook the circuit also includes a circuit which amplifies the ringing signals and simultaneously supplies the telephone line a circuit for generating timing signals synchronizes the operation of the enabling circuit and the circuit which controls the detection of the off the hook condition and also the supplying of ringing signals to the line during the times in which the sinusoidal signal generated by the exchange control components has a zero amplitude application_number US 7915787 A description the block diagram shown in fig1 of a telephone circuit in accordance with the present invention for the supply of ringing signals and the detection of the off the hook condition during ringing comprises a timing signal generator for the generation of timing signals fc having an input terminal sds for connection to exchange components are designed to generate a signal with a wave shape which is substantially sinusoidal and having a frequency identical to the frequency of the sinusoidal ringing signals to be supplied to.