A communication system capable of improving telephone communication service by effectively integrating communications with IP phones and with mobile phones. A connection controller controls a connection interface with a mobile phone and obtains the phone number of the mobile phone being connected thereto. A connect information transmitter transmits connect information including identification information and the phone number. A connect information manager stores and manages the received connect information. When a calling IP phone sends a request to call the mobile phone, a communication controller determines based on the connect information whether the mobile phone is connected to a receiving IP phone. If yes, the communication controller makes a call to the receiving IP phone to establish communication between the calling and receiving IP phones.