To provide a mechanism which, when allowing an extension telephone to receive an incoming call from an outside telephone network, enables a user to recognize a calling source on the extension telephone side by leaving information capable of specifying the calling source as call history without leaving a telephone number of the outside calling source in the extension telephone, and even when the extension telephone is lost, can reduce leakage risk of customer information.

A system number related to a calling source number received from the outside telephone network is issued, the system number is converted into a second calling source number, and then the second calling source number is transmitted to an extension telephone network.