Number of voice channels for PSTN call
Regular analogue phones require two wires to operate and by operate I mean transfer voice signals in both directions simultaneously without the earpiece you are using getting bombarded with your own amplified voice from the handset microphone.

Basically the transmit and receive amplifiers are wired as a kind of wheatstone bridge and this cancels local sounds from being locally received: -



Picture from here.

The cancellation of local transmitted voice in the earpiece is called "sidetone balancing" and relies on the transmitted signal to line producing exactly the same voltage on both inputs of the differential receiver for the earpiece. For this to work effectively, the line's characteristic impedance has to be controlled throughout the telephone network.

If you understand wheatstone bridges then you should understand this.


  Does Public Switched Telephone Network (PSTN) use two voice channels
  for a single voice call - one for each direction?


The network may convert the forward and return voice signals into two separate signals so that line amplifiers can be added when line lengths are particularly long: -



Picture from here (ELC-523).

The above circuit is one half of what is known as a telephone hybrid and here's the bigger picture that shows 2 to 4 to 2 wire amplifier: -



Picture from Elliot sound.

In detail, the hybrid can be realized with two of these although solid state versions are used in the modern world: -



Picture from Elliot sound.

The two wires entering your home are also used to transmit the ringing signal (circa 90 volts AC).

See also this answer for other details.