Abstract: A telephone unit comprising a case containing at least a microphone and a loudspeaker and carrying a switch for putting the unit in and out of circuit. The case has in the center part of one of its sides a slot and a multi-contact connector is disposed in facing relation to this slot. The contacts of the connector are electrically connected to at least some of the components within the case. The connector provides the normal connection between these components when the unit is used alone and cooperates with a secondary complementary multi-contact connector which may be plugged in the first-mentioned connector and is connected to circuits which are insertable in the circuit contained in the case by a plugging together of the first connector and second connector.