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.
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.