This is a Slackware setup, your mileage may vary.
if Mandrake hasn't munged it up, you should have a file named /etc/inittab that controls the startup / login processes. You need to uncomment one of the following lines:
#d1:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS0 vt100
#d2:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS1 vt100
if your modem is using COM1 use ttyS0, if it's using COM2 use ttyS1
the /sbin/agetty may point to a different program, such as mgetty. etc
The user must have an account set up on the server in order to login.