Call Redirect Enhancements to Support IP-to-IP Calls through the IOS Voice Gateway. The Cisco IOS Voice Gateway has been enhanced to use call redirection if an incoming VoIP call matches an outbound VoIP dial peer. The gateway sends a 300 or 302 Redirect message to the call originator allowing the originator to reestablish the call.
Two new commands allow you to enable the redirect functionality, globally or on a specific inbound dial peer. For command-level information, see the appropriate command page.
redirect ip2ip (dial-peer)
redirect ip2ip (voice service)
Try:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5012/products_feature_guide09186a0080181111.html