Digital VoIP gateways are small stand-alone appliances that allow you to convert voice media between PRI (Primary Rate Interface) connections (which run over T1/E1 lines) or BRI lines (UK) and VoIP connections. They are the most cost-effective solution for businesses with a legacy phone system infrastructure that want to connect to a VoIP network, such as a SIP trunk. They are also great for businesses with an IP-based phone system that need to connect to the PSTN.