Retrofit apparatus for being mounted on a telephone terminal block previously mounted at a subscriber's premises and for providing a demarcation point at the telephone terminal block between an incoming telephone line and a subscriber premises line previously connected through first terminal means provided on the telephone terminal block, including telephone jack means for being connected to the first terminal means; second terminal means for being connected to the subscriber premises line upon the subscriber premises line being disconnected from the incoming telephone line; and telephone plug means connected to the second terminal means and for being plugged into the telephone jack means to interconnect the incoming telephone line and the subscriber premises line and for being unplugged from the telephone jack means to provide the demarcation point and permit the plug of an operating telephone to be plugged into the telephone jack means to facilitate determination of whether a fault exists on the incoming telephone line or the subscriber premises line.
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