In a communication system, a PBX, a smartphone, and a communication adapter are connected. When detecting an event related to a call such as an incoming call and an outgoing call, the communication adapter transmits information on the event to the smartphone through BLE communication with the smartphone. The smartphone makes a notification of the occurrence of the event, based on the information on the event transmitted from the communication adapter. After the notification of the occurrence of the event, the communication adapter transmits an Invite command, 180 command, or the like to the PBX.