Existing modems and phone lines are employed to allow two or more data processing systems to communicate. The data processing systems communicate utilizing weak signals on top of existing phone line voltage levels. The signals are sufficiently small so as not to be interpreted as a receiver being lifted off hook. The weak signals may automatically adapt to changes in the phone line voltage due to a receiver be lifted off hook or an incoming call being signaled. At a sufficiently high frequency, the data processing systems may communicate while the phone lines are being simultaneously employed for conversation.