Abstract: In accordance with the present invention, plugs are provided for insertion into the conventional "310" or bantam styles of DSX telecommunications jacks to permit monitoring of both transmit and receive circuit paths without injecting noise into those circuit paths and without any risk of interrupting or breaking those circuit paths. Because these two conventional jacks differ in size, two sizes of plugs are necessary. However, both sizes of plugs are identical in operation and function. The teachings of the present invention can also be applied to plugs for insertion into other types of conventional jacks, such as standard phone jacks. In these other applications, the plug of the present invention requires only one contact for monaural applications and two contacts for stereo applications.