A VoIP-UMS accessed via a private branch exchange or an IP-phone using a Session Initiation Protocol (SIP). More particularly, the method has access the VoIP-UMS in which an initial call setup message is sent which includes call information upon accessing the VoIP-UMS for the purpose of using functions, such as voice mail or the like. When a user accesses a voice service system over a network directly from a terminal, the user accesses via an SIP server, or the user accesses via a switching system, the VoIP-UMS can discriminate connected calls easily, and determinate and respond to the calls rapidly because an INVITE message, which is a Session Initiation Protocol message attempting to access the VoIP-UMS, includes an INFO message that is a message on call information.