A communication apparatus has an off-hook detection function, and has a first electric power supply for supplying electric power to a telephone control unit for controlling a telephone apparatus, and a second electric power supply for supplying electric power to an off-hook detection unit for detecting an off-hook of the telephone apparatus. The apparatus causes the off-hook detection unit to detect an off-hook of the telephone apparatus when, from a first power mode, a second power mode, in which electric power consumption is less than in the first power mode, is transitioned into.