Telephone testing equipment, particularly a telephone subscriber test set of the type enabling the subscriber to isolate malfunction either in the household interior telephone equipment, including interior household lines and telephone receivers, or in the telephone company central office lines. A test circuit is connected to the household interior lines, so as be interposed between the telephone company central office lines and the household receiver. This test circuit includes a normally "open" test switch, a transformer and a dial tone speaker. As the test switch is "closed", the dial tone, emanating from the telephone company central office lines is amplified through the test circuit speaker. The device includes an especial extension test unit for use in households, having more than one telephone receiver.