Core Issue
Normally, the source Public Switched Telephone Network (PSTN) switch provides ringback to the calling party. However, this does not always occur and it may become the responsibility of the destination switch to provide ringback. Because the Cisco CallManager cannot provide ringback, the gateway may therefore need to be instructed to do so on its behalf.
This issue is well understood and the normal workaround is to issue the progress_ind setup enable 3 command on the dial peer pointing to the Cisco CallManager. For more information, refer to PSTN Callers not Hearing any Ringback When they Call IP Phones or IP Telephony / VoIP - Troubleshooting No Ring Back Tone on ISDN-VoIP (H.323) Calls.
In Cisco Survivable Remote Site Telephony (SRST), the concept of virtual dial-peers is introduced and these cannot be configured by the user. In addition, prior to certain levels of Cisco IOS , the progress_ind setup enable 3 command was not enabled by default. This issue is documented in Cisco bug ID CSCdw73507.
Resolution
The progress_ind setup enable 3 command has been made default on e-phone dial-peers in Cisco SRST in the images noted in the bug including Cisco IOS Software Release 12.2(8.5)T and Cisco IOS Software Release 12.2(2)XT03.
