The VoIP gateway is a device that allows VoIP calls to be routed over public switched telephone networks
VoIP Gateway
(PSTN). The gateway can route either incoming or outgoing calls to the PSTN. This can help to save considerable telephone costs between offices in different cities, states, or countries by routing PSTN calls to the cheaper VoIP network. The VoIP gateway can be a separate device or a card installed in a computer. These gateways have many features, such as, management and billing. They are also scalable for high density applications and changing architectures.
Many vendors are coming out with new VoIP gateway to meet the increasing business demands and interoperability to connect with a variety of communication offerings.
The VoIP Adapter website has additional information that you might find helpful.