Re: Yeah right @Hoppy
"If I remember correctly, ISDN specified a 144Kb/s link, which could carry a 2 voice calls, each using 64Kb/s, and a 16Kb/s signaling channel."
That's correct, though those 2 64k channels could carry everything, even non-voice. It's signalled via the "bearer-capability".
"Also IIRC from my POTS training, analog phone lines used to have a filter at 8KHz, which was regarded as plenty high enough to carry voice communications."
Well there were early very long lines which used indutances on the lines which acted as a low pass filter, but extended the reach. I don't know exactly where that filter was.
However the actual limit was when carrier-wave systems were introduced shortly before WWII. Those stacked voice channels in frequency so a single coaxial cable could carry dozends of voice channels. So obviously you had steep filters to only give you a passband of 300-3400 Hz so they could stack more channels. (in fact there are reports about the stacking being changed during the day, so at night you actually got wider channels) Back in the days however you were likely to even get less if you had a worn out microphone capsule.
As for digital telephone networks they decided to use a sampling rate of 8000Hz as this allowed for affordable analog filters on both sides and was well withing the technical capabilities of the 1960s. The codec they used was G.711 which could be implemented fairly easily as it could be done by having some analogue circuity and an 8-Bit A/D converter.
ISDN actually had a special bearer capability for G.722 encoded audio which allows for frequencies of up to about 7.2 kHz to be transmitted over a single 64k channel. This caused quite some hype at radio stations, but ultimately fell into obscurity.