Abstract: An apparatus communicates a diagnostic device with a telecommunications service source, or alternatively with a telecommunications service subscriber device, through a remote network unit. The apparatus includes a test plug structure receivable in operative engagement with a panel of over-voltage protection receptacles in the remote network unit. The apparatus further includes an engagement structure which connects the test plug structure with the diagnostic device in a plurality of selectable alternative conditions. These include a first condition in which the engagement structure communicates the diagnostic device with the service source through the test plug structure, and a second condition in which the engagement structure communicates the diagnostic device alternatively with the subscriber device through the test plug structure.