When switching from one telephone line to another, a private branch exchange $($PBX$)$ or central office must handle the BORSCHT function he line, which includes features such as

A$.$ alerting central office to on$-$ and off$-$hook conditions, and modem transmissions.

$3.$ ringing signal, battery power, and protection against overvoltage.

F$.$ battery power, frequency reuse, and subscriber loop interface circuit $($SLIC$).$

p$.$ transmitted signals, multiplexing, and PCM$.$