I seem to also recall that you couldn't use a 56k modem unless the
far-end was digital.
Exactly so - the connection to the telephone network needed to be as "clean" as possible for the modem to achieve the best rate, which was only possible with DSPs talking PCM directly into the PSTN to synthesise the perfect analogue representation of the signal.
56k modems were asymetric - the uplink was 33.6 (V.90) as that's the best you could get whistling up an analogue line.
I'm guessing that if the modem industry didn't target the US market first, those modems would have been 64k download.
AledÂ