They were, they got compressed with G.711 or G.722.
In fact, that's why your 56kbps modem would often fall back to 38.4kbps or 28k8, until the phone company installed a fancy new exchange that demodulated the 56kbps stream and didn't compress it. The 56kbps was also due to sampling limits/bandlimiters, on the same copper line you could also get a fully digital ISDN line that did 64kbps. (And if they remove all the filters and band limits, you can reach DSL speeds.)
There's nothing inherently special about voice-compression compared to any other kind of interference/distortion you can get on an analogue line.
In fact, that's why your 56kbps modem would often fall back to 38.4kbps or 28k8, until the phone company installed a fancy new exchange that demodulated the 56kbps stream and didn't compress it. The 56kbps was also due to sampling limits/bandlimiters, on the same copper line you could also get a fully digital ISDN line that did 64kbps. (And if they remove all the filters and band limits, you can reach DSL speeds.)
There's nothing inherently special about voice-compression compared to any other kind of interference/distortion you can get on an analogue line.
Also, faxes still work?