In the web-interface go to: Settings - Telephony - VoiP - RTP
You need to enable the feature via the Switch "Enable RTCP-XR"
Enter the RTCP-XR server address and the RTCP-XR server port.
This will enable the device to send an RTCP-XR report after an call.
To make sure that the report also has the RTCP information, do not forget to enable "Use RTCP", else the report will be empty.