Most modems have two RJ-11 telephone connectors, labeled LINE (green) and PHONE.
To set up the modem, connect a telephone cable from a telephone service outlet to the LINE connector. Optionally, connect a telephone set to the PHONE connector. A connected telephone operates normally, even if the computer is powered off.
You cannot use this telephone if the modem is connected. Attempting to use it while online may drop the modem connection.