To stop an IP address from coming in: 
ipchains -A input -s  -j DENY 

Examples
 
To stop 24.100.50.25 from coming in,
append to the Input chain the following rule: 
Translated: Do not allow ip address 24.100.50.25 into the box, jump to DENY
 
ipchains -A input -s 24.100.50.25 -j DENY 


 
If you don't want to allow your box to access a certain ip address, 
then append to the Output chain the following rule: 
Translated: Do not allow access to ip address 24.100.50.25, jump to DENY
 
ipchains -A output -d 24.100.50.25 -j DENY 

0)  Verify attached modem is set up correctly

    ls -a /dev | grep -i modem

1)  Modify /etc/inittab, add a line similar to this:
    ( example is for COM2 )

    RH < 7 
    s0:235:respawn:/sbin/uugetty ttyS1 F57600 at386
    
    RH 7.1  (need to install mgetty rpm)
    s0:235:respawn:/sbin/mgetty ttyS1 F57600 at386    
    

2)  Add a user:  pppuser

    useradd -s /usr/local/bin/ppp-login pppuser


3)  Create the login script for pppuser

    #!/bin/bash
    
    rmtip=192.168.99.201
    locip=192.168.99.202
    
    exec /usr/sbin/pppd -detach silent modem crtscts $locip:$rmtip    


4)  Chmod pppd, as root

    chmod 4777 pppd


5)  Change /etc/ppp/options.ttyS1  (example is for COM2)

    lock
    noauth


Windows Client Machine

Create a connection, in the dial-up networking panel.  Try it out;
if unsuccessful, try using the scripting option, with step by step
processing.  This usually will lead you to the problem.

Check /var/log/messages on the Linux machine for more information.