A communication terminal unit has a system control section including a call destination specification information analysis section that analyzes a destination number which is assigned to a communications terminal unit at the other end and input at the time of origination of a call. On the basis of a result of analysis of the destination number performed by the call destination specification information analysis section, a call origination network determination section automatically determines to originate a call over an IP network or over a public network through use of the destination information that has been subjected to analysis and through use of selected network setting information stored in a selected network setting storage section. Selected network setting information required to automatically determine to originate a call over the IP network or over the public network is stored in the call origination network determination section.