Abstract: A tone ringer for a telephone set, responsive to both low-voltage call signals of relatively high frequency and high-voltage call signals of a relatively low frequency, comprises an active filter connected across a line loop for transmitting the low-frequency call signals in attenuated form and the high-frequency call signals in amplified form to a storage capacitor in a signal detector which in the presence of either type of call signal accumulates a sufficient charge to activate a threshold circuit in that detector. A normally blocked current source, connected across the line loop via a diode bridge, is unblocked by this threshold circuit to energize a tone generator working into an electro-acoustic transducer.

Abstract: A receiving amplifier AR and a transmitting amplifier AT at a telephone-subscriber station have operating circuits in respective diagonals A-D and B-C of a bridge circuit whose four impedance arms are formed by two fixed resistances Z.sub.1, Z.sub.2, a variable resistance Z.sub.3 and the resistance Z.sub.4 of a line linking the subscriber station with an associated central office. The variable bridge resistance Z.sub.3 includes a plurality of resistors selectively insertable under the control of a voltage sensor and stabilizer SL connected across the transmitting diagonal B-C which also biases a variable-gain input stage of each amplifier AR, AT to adapt it to the line resistance. A final stage of the transmitting amplifier AT has an input connected via a negative-feedback resistance ZA to one of its output terminals whereby the effective resistance of this amplifier is reduced to a small fraction of the two bridge resistances Z.sub.1, Z.sub.3 in parallel therewith.