Image forming apparatus and communication system

An image forming apparatus, comprising:

• a voice communication device;
• a login management portion configured to perform user authentication on the basis of a user ID and allow login of a user;
• and
• a transfer control portion configured to transmit, to a predetermined call transfer control device, a transfer setting request for causing the voice communication device to change an incoming call destination of a telephone number that corresponds to the user ID of the user whose login has been allowed, wherein
• the login management portion is configured to, when a first user logs in, prohibit login of a second user different from the first user, and is configured to, when the voice communication device starts voice communication of the telephone number that corresponds to the user ID of the first user, allow login of the second user while prohibiting use of the voice communication device by the second user different from the first user.