I have never used those "56K" dial-up modems because they are asymmetric:
it's only "56K" in the downstream direction, and I oppose that on
principle. Prior to switching to my current 384 kbps SDSL (served by a
V.35 CPE device of my own design and make, see earlier messages in this
thread), I was using an always-on (immediate redial on disconnect) analog
modem connection at 31200 bps. One of the 4.3BSD-Quasijarus MicroVAXen
acted as the router, and the PPP implementation in the kernel was written
by me from scratch: the original non-Quasijarus 4.3BSD only had SLIP.