Make sure that incoming modems automatically log out the user if the telephone call is interrupted.
Make sure that incoming modems automatically hang up on an incoming call if the caller logs out or if the caller's login process is killed.
Make sure that outgoing modems hang up on the outgoing call if the tip or cu programs are exited.
Make sure that the tip or cu programs automatically exit if the user is logged out of the remote machine or if the telephone call is interrupted.
Make sure that there is no way for the local user to reprogram the modem. Disable any remote configuration or testing features.
Greet incoming connections with an appropriate banner.
Do not install call-forwarding on any of your incoming lines.
Consider getting CALLER-ID/ANI to trace incoming calls automatically. Log the numbers that call your system.
Physically protect the modems and phone lines.
Disable third-party billing and call-forwarding on your modem lines. Don't order long-distance service on modem lines that don't need it.
Consider getting leased lines, callback modems, or telephone firewalls.
Consider using separate callout telephone lines with no dial-in capability for callback schemes.
Check permissions on all associated devices and configuration files.
Consider using encrypting modems with fixed keys to guard against unauthorized use or eavesdropping.
Use a telephone scanner to search for unauthorized modems.
Consider changing your modem phone numbers periodically.