A communication terminal apparatus includes a network facsimile device having network functions, and an SIP adapter device connected to the network facsimile device to enable communication based on Session Initiation Protocol SIP for communication control. The SIP adapter device has a first IP telephone number for use in first communication defined as communication performed between a plurality of communication terminal apparatuses and a second IP telephone number for use in second communication defined as communication performed by deeming a transmission signal as a voice signal. The SIP adapter device comprises a controller for causing the network facsimile device, according to a result of selection by the network facsimile device, to implement either the first communication by using the first IP telephone number or the second communication by means of a voice coder/decoder by using the second IP telephone number.