To provide a technology which allows one telephone set to selectively use a plurality of SIP servers, for private use, business use and the like.

In the case that a call originating instruction is received from the telephone set 2, on the basis of a number sequence being designated by the call originating instruction, a telephone adapter 1 determines a SIP server of a connection destination from between a public IP telephone service SIP server 3 and an extension IP telephone service SIP server 4. The telephone adapter then transmits to the SIP server of the connection destination an INVITE request containing a connection destination number specified by the number sequence being designated by the call originating instruction, and implements a SIP call origination sequence. Therefore, one telephone set 2 can selectively use a plurality of SIP servers such as, for example, the public IP telephone service SIP server 3 for private use and the extension IP telephone service SIP server 4 for business use.