The issue here, I think, is not so much to do with digitising the analogue signal and "compressing" the data - it is to do with the resilience against packet loss and packet delay over the IP network.

The point is that IP networks were designed purely for data - not for voice comms.


Note that "PSTN" is "Public Switched Telephone Network" - which refers to a circuit-switched network specifically designed for telephone (ie, voice) traffic.

PSPDN = Packet-Switched Public Data Network.