A circuit for notifying a PBX that a data port in a telephone connected to the PBX is about to be used. The circuit comprises a current sensor for generating a signal that indicates a modem is drawing current through the data port. A large resistor limits the current being drawn through the data port to a level that does not indicate an off-hook condition to either the PBX or the modem. In response to the signal generated by the current sensor, a separate telephone circuit provides an off-hook condition that draws a first dial tone from the PBX. A microprocessor in the circuit generates a signal that causes a dialer in the telephone circuit to dial a number that notifies the PBX that the modem is attempting to access the PBX through the data port. After the PBX is notified of the imminent data port usage, it presents a second dial tone on line two which is the dial tone for an outside line. The microprocessor then activates a thyristor which electrically bypasses the resistor and allows sufficient current to be drawn by the modem to maintain the off-hook condition.