A communication apparatus, which is connected to a phone line, and to which an external telephone is connectable to. The communication apparatus includes: a line-voltage detecting unit which detects a line voltage; a communication-signal detecting unit which performs detection on a communication signal, which is input to the communication apparatus from the phone line or from the external telephone upon the external telephone is hooked up; and a control device which performs a hook-up determining process of determining that the external telephone is in a hook-up state when the line voltage detected by the line-voltage detecting unit is equal to or less than a first threshold value and the communication signal is detected by the communication-signal detecting unit.