[1] The properties of metering signals differ between the telephone administrations in various countries.
Some systems use alternating current pulses superimposed on the direct current (battery) levels of the local loop, while others may use a reversal of tip-ring polarity of the wire pair.
50 Hz pulses are applied to the telephone circuit as common-mode signals with respect to ground, as applying them differentially would make them audible for the talking parties as buzzing tones.
The pulse amplitude (RMS voltage) is high enough to distinguish them from signals induced from mains electricity.
On residential telephone lines, metering pulses are not used to generate billing records, but serve to inform the calling subscriber with a visual display in form of a counter or advancing dial connected to the telephone instrument.