The Extension Monitor is a discreet real-time telephone line monitoring device which works via any phone jack on the system. Unlike monitoring using a regular telephone, this device is transparent on the line; there are no "clicks", volume decreases, or voltage drops as would occur if a telephone was used for this purpose. The user can silently monitor both sides of the telephone conversation through the unit's earpiece. WARNING: this product is to be used only in a legal and lawful manner in compliance with all applicable laws and F.C.C. regulations.