Core Issue
This is an example of the issue:
Phone A calls phone B, which is registered to Cisco CallManager Express and forwards calls to a Cisco Unity Express. If the Cisco Unity Express is unavailable or is in the registering state, phone A does not hear a ringback tone or a busy tone.
Resolution
Phone A does not hear a ringback tone because it is still in the call transfer process to the Cisco Unity Express voicemail. Since the Cisco Unity Express is unavailable, it retries the transfer six times by default.
The sip-ua command can be configured in the Cisco CallManager Express in order to reduce the number of retries.
This is a sample configuration:
configure terminal
sip-ua
retry invite 1
Refer to Cisco CallManager Express/Cisco Unity Express Configuration Example for more information.
