Traditional digital voice telephony is astonishingly good compared to anything we now use on mobiles. The problem with mobile has never been with the bandwidth, it's the packet loss. You can have as wide a band as you want but if you are dropping half the frames or they arrive at random times then it still sounds like shit. Isochronous circuit switching basically works perfectly, as long as the path can be established (busy signal is the failure mode here).

On VoIP with a good low loss path you can get amazing quality too. Skype for example sounds even better than digital circuit switched telephony.