[Station X]

In theory these diodes are not needed, as the receiver is shorted out by the auxiluary dial springs during dialling. However there is always some slight pulsing noise due to induced currents through the receiver. Also if the auxiluary dial springs are dirty or maladjusted the shorting is ineffective.

Diodes don't conduct until the voltage across them reaches a certain value. This value is low enough to permit speech currents through the receiver, but high voltage pulses are short circuited. Two diodes connected back to back are needed as speech currents and high voltage pulses can be of either polarity.

Personally I like the sound of dial pulses. It's all part of the charm of an old phone.