A communication system capable of preventing data transmission to a terminal not desired by the user, and setting a redirect destination of data from a terminal desired by the user. A SIP server transmits address information of a first terminal to an external apparatus when the SIP server receives a connection request to the first terminal from the external apparatus. The SIP server determines whether or not to permit a second terminal to be set as a redirect destination of the connection request when it is requested by the user to set the second terminal as the redirect destination, and sets the second terminal as the redirect destination according to the determination. Then, the SIP server transmits address information of the second terminal to the external apparatus when it receives the connection request in a state where the second terminal is set as the redirect destination.