You can force the gateway to generate a ring back by issuing the progress_ind setup enable 3 command. Configure this on the VoIP pointing to the Cisco CallManager.
!
dial-peer voice 500 voip
destination-pattern 5...
progress_ind setup enable 3
!-- Forces ring back tone for this peer.
session target ipv4:10.200.73.15
codec g711ulaw
!Note: In case of multiple VoIP peers that can be matched, make sure to configure this on all of them.
This URL should help you :
http://www.cisco.com/en/US/tech/tk652/tk653/technologies_tech_note09186a0080111b80.shtml