A method uses circuitry in a data interface that provides telephony service over a high-speed network to detect the presence of a secondary source of VBH battery voltage on a tip-ring system. If a detected source other than the data interface is impressing battery voltage on the tip-ring system, the circuitry disconnects the data interface's battery source and produces a message that another source is VBH. The determination that another source is present may be based on a threshold value to account for noise. The message is used to produce an alarm at a user interface device. The alarm may be a light indication mounted on the data interface. The alarm may also be a web page that is retrieved from a memory device and displayed on a PC, a PDA or the data interface. The alarm may be provided locally and remotely to a provider's server.