Abstract: A unit for providing heating, ventilation, and/or air conditioning (HVAC, 15) is controlled by one or another thermostat (17a, 17b) in response to a switch (21) the setting of which is controlled by dual tone multiple frequency signals sent from a regular telephone (26) over ordinary phone lines (27). The functions of ring detection and answering a call (28) as well as tone detection (30) may be provided by selected modules (34-38) or by an answering machine (83). A switch used to select one of the thermostats may comprise a stepping relay (21a) a bistable toggle relay (21b), or a bistable relay 21c. The phone control switch apparatus may selectively apply power to an outlet (91) for running a self-contained HVAC such as an air conditioner (94) or a heater (95).