Hi,
I would recommend you first read man pppd and netbsd networking 'howto'
on ppp  setup. mgetty+sendfax is the logical choice for dialin server,
this package is found in /usr/pkgsrc/comms directory. You should first
set up /etc/ppp directory with dialout  'options' make sure this works
before trying dialin with mgetty.
-Mel