SIP/PSTN gateway(A device that translates signaling and media in different formats between the Enterprise Voice infrastructure and a public switched telephone network (PSTN) or private branch exchange (PBX))
IP/VoIP gateway(A third-party hardware device or product that connects a legacy PBX to a LAN. An IP/VoIP gateway translates or converts TDM or telephony circuit-switched protocols to packet switched protocols that can be used on a VoIP-based network)
WAP gateway(A software usually on a mobile operator's network that converts desktop webpages to a mobile format. Helpful for older phone models that can't render the full desktop page)
