The RS-232 Data Terminal Ready signal is lowered when the computer wishes the modem to hang up. The computer wishes to hang up when people have ended their login session ends or when they fail to respond to the login: prompt.
Using a modem cable that has DTR wired and a modem that is configured to obey DTR is essential to prevent denial of service attacks upon the access to the console.
Without DTR a caller can simply hold the modem line open, denying system administrators access to the console.
The RS-232 Data Carrier Detect signal is lowered when the user hangs up.
Using a modem cable that has DCD wired and a modem that is configured to assert DCD is essential to prevent people dialling in after a user has hang up and from carrying on their session.
Without DCD the session is not cleared when an accidental disconnection occurs. This allows any subsequent caller to resume the previous session. The machine is totally compromised if the previous user was root.