Abstract: A portable instrument for testing the integrity and measuring the attenuation of a subscriber line of a telecommunication system comprises a metering unit joined to a plug by which an associated service telephone can be connected across a line to be tested. The metering unit includes a pilot-tone generator as well as a digital attenuation indicator and is provided with three pushbuttons, two of which disconnect the service telephone from the line on being respectively actuated for transmitting the locally generated pilot tone to the line or feeding an incoming pilot tone to the attenuation indicator. Operation of the third pushbutton connects the local generator to the indicator for calibration purposes. Any of these pushbuttons, when depressed, supplies operating current from a normally open-circuited local battery to all other components of the unit.