A VoLTE call is read across three layers at once: the radio, the IMS SIP signaling and the RTP transport of speech. Reading a call in a device trace means following these three threads on a single timeline, through to the release cause.
1. Call setup, including preconditions
INVITE with an SDP offer and quality-of-service preconditions, 100 Trying, 183 Session Progress, PRACK, then UPDATE once the dedicated voice bearer is established, 180 Ringing, 200 OK and ACK. Setup time is measured from the INVITE to the 180 Ringing; a late 183 or a missing UPDATE points to the establishment of the dedicated bearer rather than to the IMS.
2. Release causes
- A BYE often carries a Reason header: Q.850 cause 16 is a normal call clearing.
- RELEASE_CAUSE 2 (RTP/RTCP time-out) or 3 (media bearer or QoS lost) indicates a drop related to the transport or to the loss of the voice bearer.
- A setup failure is read in the final response, 4xx or 5xx, and in its cause.
- On the radio side, a radio link failure followed by a rejected re-establishment often precedes the loss of the voice bearer.
3. Perceived quality
Without an audio recording, quality is estimated from the transport: RTP packet loss, jitter and delay, converted into a MOS by the ITU-T E-model (G.107). Standardized perceptual methods (ITU-T P.863) give a finer score but require recording the audio signal.
4. Cross-checking with the radio
SINR, BLER, transmit power, radio link failures and cell changes during the call: most voice degradations have an identifiable radio cause. A MOS that drops while the transmit power is pinned at its maximum points to the uplink.
5. Audit KPIs
- IMS registration success rate and delay
- Call setup success rate and setup time
- Call drop rate and release cause breakdown
- Estimated MOS, RTP packet loss and jitter
- Correlation with radio events