H.323 Gateway
An H.323 gateway is an optional type of endpoint that provides interoperability between H.323 endpoints and endpoints located on a switched-circuit network (SCN), such as the PSTN, the ISDN or an enterprise voice network. Check http://docwiki.cisco.com/wiki/Cisco_IOS_Voice_Troubleshooting_and_Monitoring_--_H.323_Gateway_Troubleshooting