Session Initiation Protocol (SIP), a protocol used in VoIP (Voice over IP) communications, enables a caller to send a SIP message to a callee with his/her UserID concealed from the callee, which in turn, prevents A SIP server from making an attempt to manage the caller. If the same IP Telephone address is used whenever the IP call is made, a third party may guess the IP address easily during conversation. The SIP message sent by the caller is converted and conversion tables are managed by the SIP server. The IP Telephone modifies its IP address for each IP call. Thus, the caller can make a call to the callee with his/her UserID concealed from the callee using the SIP server with a message conversion function and communications carriers having SIP servers installed to manage the callers using their conversion tables. It is difficult to any malicious third party to guess the IP Telephone address, because the IP Telephone address is modified for each call.