How to Use This Guide
Answer each question in the decision tree based on what you observe in the DIAG log or the device behavior. The tree will guide you from symptom to root cause in 5 steps or fewer. Each outcome links to the relevant protocol guide or diagnostic procedure.
VoLTE Call Drop: Interactive Diagnosis
A VoLTE call drop has at least six distinct root causes — and each requires a different fix. This decision tree guides you from the observable symptom to the specific log evidence that confirms the root cause, without reading hundreds of lines of DIAG output.
Start: When did the drop occur?
Check the IMS OTA log. Was a SIP 200 OK exchanged between INVITE and the moment of failure?
Root Cause Summary
| Root Cause | Key Log Evidence | Remediation |
|---|---|---|
| Too-late handover | A3/A5 MeasurementReport without subsequent HO command, then RLF | Reduce TTT, increase neighbor list coverage |
| Coverage gap | RSRP < -115 dBm before RLF, no neighbor cells in MeasurementReport | Add cell or repeater in dead zone |
| SRVCC failure | PS-to-CS HO command present, no CS RTP establishment in Tv300 | Check MSC/BSS SRVCC configuration, STN-SR provisioning |
| IMS deregistration | Missing REGISTER refresh before network BYE, QCI 5 bearer loss | Extend REGISTER expiry, improve QCI 5 bearer continuity in HO |
| QCI 1 bearer drop | Deactivate EPS Bearer (QCI 1) without IMS BYE or RLF | Check PCRF session continuity, PGW inactivity timers |
| Remote party disconnected | SIP BYE From = callee URI (not network address) | Check callee's device behavior and network conditions |
Frequently Asked Questions
In field deployments, VoLTE call drops most commonly result from: (1) handover failures — the device moves out of coverage during a handover and the radio link fails before the call is transferred; (2) IMS registration expiry — the SIP REGISTER expires during the call due to an IMS APN bearer issue, causing the S-CSCF to release the call; (3) QCI 1 bearer loss — the dedicated voice EPS bearer (QCI 1) is released during the call due to a PCRF or PGW failure. Each root cause produces a different pattern in the DIAG log and requires a different remediation path.
A call drop occurs after the call is established (200 OK and ACK exchanged). It appears in the DIAG log as a mid-call SIP BYE (sent by the network or the remote party), an RTP stream interruption without a SIP BYE (radio link failure), or an IMS deregistration event during the call. A call setup failure occurs before the 200 OK — the INVITE receives a 4xx/5xx response or the timer B expires without a final response. The timestamp of the failure relative to the SIP 200 OK determines which category applies.
SRVCC (Single Radio Voice Call Continuity) transfers an active VoLTE call to the CS (circuit-switched) domain when the UE moves out of LTE coverage into 2G/3G. For SRVCC to work: (1) the device must support SRVCC (ue-SRVCC capability), (2) the network must support PS-to-CS handover (STN-SR and C-MSISDN provisioned in HSS), (3) LTE coverage must drop to the A2 threshold before SRVCC is triggered. Without SRVCC, leaving LTE coverage during a VoLTE call always produces a call drop. SRVCC failure (handover command sent but RTP not re-established on CS) also produces a call drop.
A VoLTE call drop caused by radio link failure shows in the DIAG log as: (1) LTE RRC log: rapid RSRP/RSRQ degradation, followed by N310 consecutive out-of-sync indications, T310 timer expiry, and an RRC Connection Re-establishment Request or RLF report; (2) IMS OTA log: no SIP BYE received before the RTP stream stops — the call ends without a graceful SIP session termination; (3) EPS bearer log: QCI 1 bearer (voice) and QCI 5 bearer (IMS) both released at the same timestamp as the RLF. This pattern is distinct from a network-initiated call drop, which shows a SIP BYE from the network before the bearer loss.