So basically callers over the SIP trunk does not hear ringback. You can try this
Navigate to System>>service parameters>>Call Manager
Under Send H225UserInfoMessage parameter, select "Use ANN for ringback" and make sure that the SIP trunk has an MRGL with the access to an Annunciator. You can also try taking packet capture on the CUCM when the user has called in and is getting transferred. Check whether CUCM is generating ringback or not. This would help us isolate the issue further.
Thanks for the response! That actually helped my isolate the problem...actually, problemS!
1. Hit bug CSCtl21854. Updated IOS and ring back on initial hold works. After transfer initiator dropped off (blind transfer), still got the silence.
2. Network locale set for WRONG country. I am in Canada and the default locale was set to Canada BUT the Canada locale was not installed. Installed Canada locale, rebooted cluster and all is well again.