A telephone number research device configured to be connected via a next-generation network, NGN, to a telephone (50a, 50b, 60) connected to at least a destination network that is an IP multimedia subsystem, IMS, network that supports IPv6 or a voice over IP, VoIP, network that does not support IPv6, the device comprising: a message exchange controller configured to add a first session description protocol, SDP, parameter to an INVITE request for the telephone (50a, 50b, 60) to be researched, send the INVITE request to the destination network selected by the NGN, and receive a first response message from the destination network; and a telephone number research determination unit configured to determine a usage status of a telephone number of the telephone (50a, 50b, 60) based on the first response message, wherein the first SDP parameter includes at least an IPv6 network protocol, a session border controller, SBC, has a function to convert the IPv6 network protocol to an IPv4 network protocol suitable for the destination network when the first SDP parameter is sent from the NGN to the destination network, characterized in that the message exchange controller is further configured to: determine whether the first SDP parameter has been converted based on a description of the first SDP parameter, determine that the destination network is the VoIP network when the first response message is "200 OK" and the first SDP parameter has been converted, and determine that the destination network is the IMS network when the first SDP parameter has not been converted and the first response message includes a first error code, wherein the first error code is error code 488. a message exchange controller configured to add a first session description protocol, SDP, parameter to an INVITE request for the telephone (50a, 50b, 60) to be researched, send the INVITE request to the destination network selected by the NGN, and receive a first response message from the destination network; and a telephone number research determination unit configured to determine a usage status of a telephone number of the telephone (50a, 50b, 60) based on the first response message, wherein the first SDP parameter includes at least an IPv6 network protocol, a session border controller, SBC, has a function to convert the IPv6 network protocol to an IPv4 network protocol suitable for the destination network when the first SDP parameter is sent from the NGN to the destination network, characterized in that the message exchange controller is further configured to: determine whether the first SDP parameter has been converted based on a description of the first SDP parameter, determine that the destination network is the VoIP network when the first response message is "200 OK" and the first SDP parameter has been converted, and determine that the destination network is the IMS network when the first SDP parameter has not been converted and the first response message includes a first error code, wherein the first error code is error code 488. determine whether the first SDP parameter has been converted based on a description of the first SDP parameter, determine that the destination network is the VoIP network when the first response message is "200 OK" and the first SDP parameter has been converted, and determine that the destination network is the IMS network when the first SDP parameter has not been converted and the first response message includes a first error code, wherein the first error code is error code 488.