In order to resolve this issue, set the Disable Alerting Progress Indicator service parameter to False in Cisco CallManager. This can be done when you log into the Cisco CallManager Admin page and complete these steps:
Go to the Service menu and select Service Parameters from the Cisco CallManager Administration page.
Choose the Publisher CallManager Server and Cisco CallManager service.
Scroll down to Disable Alerting Progress Indicator from the Clusterwide Parameters (Device - PRI and MGCP Gateway) section. Set this parameter to False and click Update.
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a0080094c33.shtml#ringback_pstn
http://www.cisco.com/en/US/products/hw/switches/ps2246/products_configuration_guide_chapter09186a00800ea889.html
http://www.cisco.com/en/US/tech/tk652/tk653/technologies_tech_note09186a0080111b80.shtml#fourth