The Interworking Signaling Enhancements for H.323 and SIP VoIP feature enables VoIP networks to properly signal the setup and tear-down of calls, including generating in-band tones and announcements when needed at the originating or terminating switch. When a tone (for example, ringback, busy, reorder) or announcement (for example, "The number you have dialed is no longer in service") is played at the destination switch, the backward voice path from the called party to the calling party is cut-through early, so that the calling party can hear the tone or announcement. To prevent fraudulent calls, the voice path is cut-through in both directions only after the Connect message is received from the destination. The call progress indicator, which signals the availability of in-band communication, is carried end-to-end as required when interworking with ISDN and CAS protocols.

These enhancements prevent unexpected behavior such as early alerting (when an Alert message is returned immediately after a Call Proceeding message is sent), to ensure that the calling party does not hear conflicting call progress information such as a ringback tone followed by a busy tone, and does not miss hearing a tone or announcement when one should play. In addition, support for network-side ISDN and reducing the risk of speech clipping is addressed.

This feature set provides:

End-to-end transport of Progress message with progress indicator

Generation of in-band progress tones and announcements at appropriate switch

Configuration of progress indicator information element (IE) at the H.323 gateway

Cut-through of voice path at the appropriate point of a call

Support for network-side ISDN, including disconnect with locally generated tones

Initiation of H.245 signaling at originating gateway when Call Proceeding message is received

Support for SIP 183 Session Progress message for Early Media cut-through

This URL should help you:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5013/products_feature_guide09186a0080080e2d.html