First I wanted to say this is not another how to setup your dialup connection to the ISP.
This doesn't pretend to be a complete howto but I've seen quiet a few posts on this topic in Ubuntu forums unanswered, and it took me a long time to figure it out myself. I'm not an expert on this but it worked for me and hopefully will work for you or at least will get you started. For those who interested in setting up a callback server this is the first step, you will have to edit some more config files, but you will have to do this first. Links to more info and credits are at the end.

Part I . The short version on how to get remote access to your Ubuntu box via modem. Install mgetty
Code:
sudo apt-get install mgetty
Add a line at the end of file /etc/inittab
Code:
S0:2345:respawn:/sbin/mgetty ttyS0
If your modem is on COM1.
Initialize init by typing
Code:
sudo init q
Now from Win you can use HyperTerminal to connect to your Ubuntu box with your user name/pass.

For those who want more here is a real deal

Part II

The problem: Establish PPP connection to Ubuntu server from a remote location using a modem and to share internet connection.
Laptop modem<> Server modem <> Ethernet Router <> Cable/DSL modem <> Internet

Assumptions: drivers for your modems already installed and modem is at ttyS0. I have an external modem attached to COM1 - ttyS0, COM2 will be ttyS1. I didn't have X installed so I was using nano to edit conf files, you can use gedit if you have X installed or any other editor. Make sure your are familiar with the interface of the editor before you start modifying files. Make sure you create backups of the files before you modify them. You will need to use sudo or login as root for most of the tasks. If you don't know how to configure your dial up on your client look for a dial up howto or Ubuntu docs. I also assume that you already have your local nework including routers properly configured and have an access to the Internet from your Ubuntu server.

Part II.a - Dial in configuration

1. If not already installed use synaptic or apt-get to install ppp and mgetty packages. ppp should be already installed by default so
Code:
sudo apt-get install mgetty
2. Create a group ppp by adding a line in file /etc/group
Code:
ppp:x:1001:
3. Create a new user "pppuser" or whatever you will use for your dial in connection and assign a password by using
Code:
sudo adduser pppuser
edit file /etc/passwd or use
Code:
sudo vipw
to change entry for pppuser to
Code:
pppuser:x:1001:1001:,,,:/home/pppuser:/usr/sbin/ppplogin
4. Add a line to the file /etc/inittab
Code:
S0:2345:respawn:/sbin/mgetty ttyS0
for modem on ttyS0. Or
Code:
S1:2345:respawn:/sbin/mgetty ttyS1
for modem on ttyS1 That will let mgetty to accept incoming calls

5.Make a new file /usr/sbin/ppplogin and add the following in there
Code:
#!/bin/sh
#/etc/ppp/ppplogin
# PPP login script
mesg n
stty -echo
exec /usr/sbin/pppd -detach modem debug crtscts
6. Set access to the ppplogin file and etc/ppp directory
Code:
chmod 750 /usr/sbin/ppplogin
chown root:ppp /usr/sbin/ppplogin
chmod 775 /etc/ppp
chown root:root -R /etc/ppp
7. Restart init by typing
Code:
init q
If you're use external modem it should be on before that.

8. Open file /etc/mgetty/login.config Comment out everything in there and add a line
Code:
/AutoPPP/ - a_ppp /usr/sbin/pppd file /etc/ppp/options
9. Open file /etc/ppp/options and make sure these lines are uncommented. If anything else is uncommented it probably should be commented.
Code:
        -detach
	 asyncmap 0
	 modem
	 crtscts
	 proxyarp
	 lock
	 require-pap
	 refuse-chap
	 ms-dns 192.168.1.1 #put your dns server ip here
	 usepeerdns
In my case the ms-dns entry had an ip of my router, if you using Linksys router it's 192.168.1.1 by default unless you changed it.

10. Create a file /etc/ppp/options.ttyS0 for the modem on ttyS0 and add following in there
Code:
192.168.1.3:192.168.1.201
     noauth
Where first address is the address of your server for ppp connection which I think, should be different from your eth ip. The second address is the address that will be assigned to the client when connection is established. It will probably make life easier, unless you know what you doing, if all those addresses on the same subnet as your other computers on the network. (ip starts with the same 192.168.1.x numbers)
You can substitute
Code:
noauth
for a
Code:
debug
line, this way it will log some info about you connection in a syslog.

11. Edit file /etc/ppp/pap-secrets
find a line after
Code:
# Every regular user can use PPP and has to use passwords from /etc/passwd
It should look something like that
Code:
*         hostname     ""      *
substitute hostnatname with * so it looks like that
Code:
*         *     ""      *
If you don't do that pap will not authenticate you and you'll be immediately disconnected.

Now you're able to connect using dial-up connection from you laptop or a remote office into your Ubuntu server and use ssh or putty if you're using Win.

Part II.b - Accessing internet from a remote client

I'm sure there other or better solutions to that, but that was easy enough and it worked for me.
Code:
sudo apt-get install ipmasq
Done. ipmasq automatically senses all your interfaces and initializes IP Masquerade forwarding/firewalling and allows you to connect to the rest of your network and the Internet.

I didn't have to do any more configurations for that.

Credits: Part I was inspired by debenham http://www.ubuntuforums.org/member.php?u=21302 in his answer to benson in http://www.ubuntuforums.org/showthread.php?t=65012

Part II and for more information
http://linuxgazette.net/issue77/sunil.html - Setting Up a Linux-based PPP Callback server
and http://tvilda.stilius.net/callback_en.php - Debian PPP dialin and callback server
http://www.aboutdebian.com/ - Some info on networking and other topics
http://www.debian.org/
http://qref.sourceforge.net/ - Debian reference
Those of you who are interested in setting up a callback server need to read links above. If security is a concern or you're in a business environment you should probably have a different setup.

Good luck
Alex