When Cisco CallManager is integrated with a third-party H.323 gateway/gatekeeper and IP phones attempt an outbound call through the gateway, the h225 alert message (sent by the H.323 gateway) contains the h245 address. This alert message contains no Progress Indicator (PI).
Cisco CallManager does not instruct the IP phones to play a local ringback when it receives an h245 address in the h225 alert. Once the h245 address is received in the h225 alert, Cisco CallManager starts earlier media cut-through. In this scenario, Cisco CallManager expects for ringback to be provided by the far end devices in the band. In turn, this causes the no ringback issue with a third party H.323 gateway.
This issue has been tracked in Cisco bug ID CSCsd03212.
With the fix provided in Cisco bug ID CSCsd03212, Cisco CallManager checks the PI value in the alert message before it starts media establishment. Due to the impact on backward compatibility with a Cisco IOS gateway, this fix takes the form of a service parameter. A new service parameter, CheckProgressIndicatorBeforeEstablishingMedia, has been added. The default value for this parameter is False (meaning that PI is not considered). However, to resolve the issue, change the default value to True (PI is taken into consideration before media procedures are started).
The fix for this bug is integrated in Cisco CallManager versions 4.1.3ES47, 4.2.1(ES01) and 005.000(000.358) 005.000(000.362) 005.000(001.099.033) 005.000(002.9901.006). To resolve the issue, refer to Software Downloads: Cisco Unified CallManager Version 4.2 to upgrade to one of these releases.