Digium's VoIP gateways are cost-effective, industrial grade appliances that simplify the process of deploying converged media networks. Digium's G100 and G200 VoIP Gateway are respectively single and dual port software-selectable T1/E1/PRI appliance that support up to 60 concurrent calls. 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.
In addition to the resources here, more information about Digium's Gateway products may be found at http://www.digium.com/voip-gateways