Digium's VoIP gateways are cost-effective, industrial grade appliances that simplify the process of deploying converged media networks. Digium's G100, G200, G400, and G800 VoIP Gateways are, respectively, 1, 2, 4, and 8-port software-selectable T1/E1/PRI appliances that support up to 30, 60, 120, and 240 concurrent TDM to SIP calls. Digium G080 Series VoIP Gateways are 8-port analog appliances that are provided in one of three configurations: 8 FXO, 4 FXS + 4 FXO, or 8 FXS interfaces. They are built to support TDM-to-SIP, SIP-to-TDM and SIP-to-SIP (transcoding) applications. In a TDM-to-SIP deployment the VoIP gateway significantly reduces operating costs by connecting a legacy business phone system with dynamic SIP trunking services. SIP-to-TDM deployments use the VoIP gateway to connect a modern SIP communications system with T1/E1/PRI service from legacy carriers. SIP-to-SIP deployments are also possible, with call concurrency halved as compared to operations involving both SIP and TDM interconnection.
In addition to the resources here, more information about Digium's Gateway products may be found at http://www.digium.com/voip-gateways