what is the voltage present in the telephone?

Answer Posted / prakash mp

There are no switches or user adjustments. If the coupler
is connected to a standard telephone line, there will be a
DC voltage present that is higher when the circuit is not
being used than when the line is in use. Noise on the
telephone line will not be coupled to the output of the
coupler when the phone is not in use and the voltage on the
line is greater than 35 volts DC. When the phone is in use
and the voltage drops below 35 volts DC, this circuit
permits all sounds on the couplers input to reach its output

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