Haven't tried this myself yet, but it looked interesting:
http://www.howtoforge.com/linux_dialin_server

--
==========
Todd Young

 -------------- Original message ----------------------
From: "Thomas Johnson" <tommyj27 at gmail.com>
> I've been playing around with the idea of setting up a modem on my
> home server to dial in and get access to home and the internet (via
> bluetooth/cell, yes I know how slow it is). The server in question is
> running Ubuntu 6.06 and I followed the guide at
> http://ubuntuforums.org/showthread.php?t=150339.
> 
> Thus far I have been mostly unsuccessful at even getting the modem to
> pick up, when I dial in I hear a click (presumably the modem picking
> up) followed by another click several seconds later (hanging up?). the
> mgetty log file shows activity that doesn't look correct, but I'm no
> expert so I really don't know what I'm looking at.
> 
> The only success I've had at getting the two modems to talk to each
> other was when I happened to have a 'cat /dev/ttyS0' running in
> another terminal, the modems negotiated and the client tried to
> authenticate but got nowhere.
> 
> I'm not sure what isn't set up right with mgetty, it looks pretty
> straight-forward, mgetty handles the connection, then fires up pppd to
> handle the authentication. I am copying my log output from mgetty and
> hoping that someone here has some experience that they haven't
> forgotten since dial-up became yesterday's news.
> 
> 01/25 23:40:03 yS0  mgetty: interim release 1.1.33-Apr10
> 01/25 23:40:03 yS0  check for lockfiles
> 01/25 23:40:03 yS0   checklock: stat failed, no file
> 01/25 23:40:03 yS0  locking the line
> 01/25 23:40:03 yS0   makelock(ttyS0) called
> 01/25 23:40:03 yS0   do_makelock: lock='/var/lock/LCK..ttyS0'
> 01/25 23:40:03 yS0   lock made
> 01/25 23:40:03 yS0   tio_get_rs232_lines: status: RTS CTS DSR DTR
> 01/25 23:40:03 yS0  lowering DTR to reset Modem
> 01/25 23:40:04 yS0   tss: set speed to 57600 (10001)
> 01/25 23:40:04 yS0   tio_set_flow_control( HARD )
> 01/25 23:40:04 yS0   waiting for line to clear (VTIME=1), read:
> 01/25 23:40:04 yS0  send: \dATQ0V1H0[0d]
> 01/25 23:40:04 yS0  waiting for ``OK''
> 01/25 23:40:04 yS0   got: ATQ0V1H0[0d]
> 01/25 23:40:04 yS0    CND: ATQ0V1H0[0d][0a]OK ** found **
> 01/25 23:40:04 yS0  send: AT[0d]
> 01/25 23:40:04 yS0  waiting for ``OK''
> 01/25 23:40:04 yS0   got: [0d]
> 01/25 23:40:04 yS0    CND: OK[0a]AT[0d]
> 01/25 23:40:04 yS0    CND: AT[0d][0a]OK ** found **
> 01/25 23:40:04 yS0   waiting for line to clear (VTIME=3), read: [0d][0a]
> 01/25 23:40:05 yS0   removing lock file
> 01/25 23:40:05 yS0  waiting...
> 01/25 23:45:29 yS0    select returned 1
> 01/25 23:45:29 yS0   checking lockfiles, locking the line
> 01/25 23:45:29 yS0   makelock(ttyS0) called
> 01/25 23:45:29 yS0   do_makelock: lock='/var/lock/LCK..ttyS0'
> 01/25 23:45:29 yS0   lock made
> 01/25 23:45:29 yS0  wfr: waiting for ``RING''
> 01/25 23:45:29 yS0   got: [0d][0a]NO CARRIER
> 01/25 23:45:29 yS0  wfr: found action string: ``NO CARRIER''
> 01/25 23:45:29 yS0   wfr: rc=-1, drn=0
> 01/25 23:45:29 ##### failed A_FAIL dev=ttyS0, pid=4763, caller='none'
> 
> --
> 01/25 23:45:29 yS0  mgetty: interim release 1.1.33-Apr10
> 01/25 23:45:29 yS0  check for lockfiles
> 01/25 23:45:29 yS0   checklock: no active process has lock, will remove
> 01/25 23:45:29 yS0  locking the line
> 01/25 23:45:29 yS0   makelock(ttyS0) called
> 01/25 23:45:29 yS0   do_makelock: lock='/var/lock/LCK..ttyS0'
> 01/25 23:45:29 yS0   lock made
> 01/25 23:45:30 yS0   tio_get_rs232_lines: status: RTS CTS DSR DTR
> 01/25 23:45:30 yS0  lowering DTR to reset Modem
> 01/25 23:45:30 yS0   tss: set speed to 57600 (10001)
> 01/25 23:45:30 yS0   tio_set_flow_control( HARD )
> 01/25 23:45:30 yS0   waiting for line to clear (VTIME=1), read:
> 01/25 23:45:30 yS0  send: \dATQ0V1H0[0d]
> 01/25 23:45:31 yS0  waiting for ``OK''
> 01/25 23:45:31 yS0   got: ATQ0V1H0[0d]
> 01/25 23:45:31 yS0    CND: ATQ0V1H0[0d][0a]OK ** found **
> 01/25 23:45:31 yS0  send: AT[0d]
> 01/25 23:45:31 yS0  waiting for ``OK''
> 01/25 23:45:31 yS0   got: [0d]
> 01/25 23:45:31 yS0    CND: OK[0a]AT[0d]
> 01/25 23:45:31 yS0    CND: AT[0d][0a]OK ** found **
> 01/25 23:45:31 yS0   waiting for line to clear (VTIME=3), read: [0d][0a]
> 01/25 23:45:31 yS0   removing lock file
> 01/25 23:45:31 yS0  waiting...
> 
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