This amplifier circuit is used in all home telephones to amplify the signal from the line to the earpiece. The voltage is taken from the line via a bridge that delivers a positive rail, no matter how the phone wires are connected.
A transformer is used to pick off a signal from the phone line and this is passed through a 22n to the input of the amplifier. Negative feedback is provided by a 15k and 1n2 capacitor. The operating point for the amplifier is set by the 100k pot and this serves to provide an effect on the gain of the amplifier and thus the volume.