1. A method for linking an Internet protocol (IP)-private branch exchange (PBX) system with a Voice over Internet Protocol (VoIP) terminal, the IP-PBX system including a first database storing information on a phone number that is registrable at an extension and a second database storing information of a terminal registered at the extension, the method comprising steps of: checking whether or not a phone number of a VoIP terminal included in an extension register message provided from the VoIP terminal is an extension number; and when the phone number of the VoIP terminal is an extension number, storing, in the second database, extension registration request information included in the extension register message and phone number information corresponding to the phone number of the VoIP terminal stored in the first database.