A communication system comprising: a call answering center; a telephone communication network configured to connect a first mobile phone, a second mobile phone, and the call answering center in a communicable manner, the second mobile phone being configured to make a call to the first mobile phone, the second mobile phone including a display unit and an operation unit, the first mobile phone being configured to register a call answering setting including a reason for unavailability in the call answering center; wherein, when the second mobile phone makes a call to the first mobile phone, and it is determined that the call answering setting is performed, the call answering center composes a call answering message containing the reason for unavailability, characterized in that the call answering message is accompanied by a reply program having an advertisement image attached thereto in accordance with the call answering setting, and transmits the call answering message to the second mobile phone, and, upon receiving the call answering message, the second mobile phone causes the display unit to display the call answering message including the reason for unavailability, an image of the reply program displaying options for selecting a message to be sent to the first phone, wherein the options are displayed in a selectable manner, wherein the answering message including the reason for unavailability and the image of the reply program are superimposed on the advertisement image, upon the message being selected, the second mobile phone transmits the selected message to the call answering center, and the selected message is transmitted from the call answering center to the first mobile phone. a call answering center; a telephone communication network configured to connect a first mobile phone, a second mobile phone, and the call answering center in a communicable manner, the second mobile phone being configured to make a call to the first mobile phone, the second mobile phone including a display unit and an operation unit, the first mobile phone being configured to register a call answering setting including a reason for unavailability in the call answering center; wherein, when the second mobile phone makes a call to the first mobile phone, and it is determined that the call answering setting is performed, the call answering center composes a call answering message containing the reason for unavailability, characterized in that the call answering message is accompanied by a reply program having an advertisement image attached thereto in accordance with the call answering setting, and transmits the call answering message to the second mobile phone, and, upon receiving the call answering message, the second mobile phone causes the display unit to display the call answering message including the reason for unavailability, an image of the reply program displaying options for selecting a message to be sent to the first phone, wherein the options are displayed in a selectable manner, wherein the answering message including the reason for unavailability and the image of the reply program are superimposed on the advertisement image, upon the message being selected, the second mobile phone transmits the selected message to the call answering center, and the selected message is transmitted from the call answering center to the first mobile phone.