Abstract: According to one embodiment, a telephone number research device researches on the usage status of a telephone number in an IMS network, an NGN network, and an IP network without ringing an incoming call by transmitting a specific SIP message that causes call rejection using an IMS interconnection interface or the like for interconnection between carriers. The telephone number research device is connected via an NGN network to a telephone connected to a destination network that is an IMS network or a VoIP network. The telephone number research device adds a first SDP parameter to an INVITE request for the telephone before sending the request to the destination network selected by the NGN network and determines the usage status of the telephone number of the telephone based on a response from the destination network regardless of the type of the destination network.