The Whozz Calling? detects when the phone is on-hook and off-hook. This allows the unit to determine the length of each phone call and report this detailed information to the software. Telephone monitoring equipment can detect the occurrence of the On Hook and Off Hook status of the phone line by two methods. It can either sense the voltage on the phone line itself or sense current flow on a phone line passed through the equipment.

Voltages on incoming central office (CO) phone lines are consistent among all branches throughout a physical location (building). If the voltage levels are in standard range, monitoring equipment functioning in a voltage detection mode is able to sense On and Off Hook on any branch in the building. The obvious advantage to this detection scheme is that the monitoring equipment can be connected in parallel to the CO lines . Your unit comes from the factory set to voltage detection mode because, in most locations, this detection scheme works properly.

Current on phone lines only flows to the telephone device that is Off Hook. Monitoring equipment that senses On and Off Hook by means of current flow must be connected between the phone company’s incoming line and the device(s) that goes Off Hook. The current will flow through the monitoring equipment to the Off Hook device. With this detection scheme, any telephone device that makes or receives phone calls must be attached in series (i.e. to the output jacks) “behind” the monitoring equipment.