8.3.7. Ring-back is Played on Top of Announcement or Voice 
In some cases ring-back is played at the same time as an announcement (e.g., voice mail prompt) or voice from the called party.
This condition may last for a few seconds or for the entire duration of the call,
and usually
occurs when there are delays in the network. The called device has already
answered the call and is sending voice packets (streaming RTP), but the “answer”
message (SIP 200 OK message) has not yet arrived at the device.
Please contact your system administrator to investigate the state of the Wi-Fi network.
‣ Parent topic: Section 8.3. Troubleshooting Specific Issues