Abstract: A method and apparatus is provided for analog indications of the proximity to failure of an integrated services digital network. Analog indications or figures-of-merit can include: 1) the tolerable increase in two-wire line attenuation, 2) the tolerable decrease in receiver sensitivity in a line termination unit, and 3) the tolerable decrease in receiver sensitivity in a network termination unit. The margin test apparatus of the present invention comprises a network termination unit including means for coupling a two-wire to a four-wire line, a first adjustable attenuator for adjustably attenuating a received signal, a second adjustable attenuator for adjustably attenuating a transmitted signal, and a failure detection means which monitors the received signal and is calibrated such that it detects a signal failure if the level of the received signal is less than a predetermined threshold level.