Interface circuit and main device

A port of a main device having a telephone exchange function is provided with an interface circuit to which a telephone or intercom can be connected. The interface circuit is provided with: a feed circuit for supplying required power supply voltage in accordance with a telephone or intercom connected to the port; and a state detection circuit which, if a telephone is connected to the port, detects whether the telephone is in an on-hook state or an off-hook state on the basis of the value of the input impedance of the telephone, and if an intercom is connected to the port, detects whether the intercom is in a standby state or a state in which the activation button is pressed on the basis of the value of the input impedance of the intercom.