A telephone interface protection circuit and modem incorporating the telephone interface protection circuit. The circuit includes a sensing circuit and a determining circuit. The interface protection circuit checks for an active telephone line and checks the line for over-current conditions caused when a modem designed for the public switched telephone network is plugged into a private digital telephone network. The circuit functions when the modem is operating to provide continuous monitoring. It can be built as a stand-alone device or incorporated into a modem. The circuit notifies the user of the line condition either with LEDs or with a status message sent to the user via the host computer interface.