Abstract: A telephone adaptor unit for a telephone set connectible, with other like telephone adaptor units for other telephone sets, between the respective telephone set and a common telephone line, includes a first switching device effective, when an off-hook condition is detected, to connect the respective telephone set to the telephone line and to disconnect the other telephone sets from the telephone line, an audio circuit including a speaker connected to the telephone line, a second switching device for enabling the audio circuit to receive audio signals from the telephone line and to reproduce such signals via its speaker, and a control system effective to actuate the first switching device of the other telephone adaptor units to connect their respective telephone sets to the common telephone line, and to actuate the second switching device of the other telephone adaptor units to enable the audio circuit of the other telephone adaptor units to receive audio signals from the telephone line and to reproduce such s