A voice over internet protocol (VOIP) system and method are provided. The VOIP method includes steps of generating a phone number for a first terminal device, and a domain name corresponding to that phone number, transmitting the domain name and an internet protocol (IP) address to a dynamic domain name service (DDNS) server for registration, applying the same procedure to second and third terminal devices, generating a name of a group for the first, second, and third terminal devices, generating a domain name corresponding to the name of the group, and transmitting the domain name and the IP address for registration, and acquiring the domain name for the required name of the group, thus allowing calls to be made and available within the group.