A VoIP system includes the following basic elements: terminals, which can include IP phones and software applications running on computers; the LAN infrastructure, consisting of wiring and switches; a gatekeeper which manages the terminals in a zone, providing address resolution, registration, authentication and regulating admission to the VoIP system; optional call manager software applications running on the gatekeeper or on a standalone platform; a router, which connects LAN segments together and to WAN circuits; gateways to perform format conversions between the VoIP world and the PSTN; and optional connections to the PSTN, the Internet and to managed IP WAN services or IP VPN services. A firewall is needed when connecting to the Internet.
Source: Teracom Course 301 Voice over IP, slide 3.03