Top VoLTE Interview Questions and Answers

4G LTE
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:small_orange_diamond: 10. SRVCC vs CSFB

  • CSFB (Circuit Switched Fallback): The phone switches to 2G/3G before the call starts.
  • SRVCC (Single Radio Voice Call Continuity): The call is handed over to 2G/3G during an ongoing VoLTE call.

:small_orange_diamond: 11. MME and PCRF Role in VoLTE

  • MME (Mobility Management Entity): Manages signaling and mobility for VoLTE calls.
  • PCRF (Policy and Charging Rules Function): Handles QoS and policy enforcement for VoLTE sessions.

:compass: Troubleshooting VoLTE

:small_orange_diamond: 12. Why Might a VoLTE Call Drop?

  • Weak LTE signal (poor coverage)
  • SRVCC failure during handover
  • IMS registration or signaling issues
  • SIP or RTP failures (signaling or media path)

:small_orange_diamond: 13. How to Confirm IMS Registration

  • SIP REGISTER: Verify device registration with IMS.
  • Engineering Mode: Check IMS registration status directly on the device.
  • IMS Logs: Review logs for SIP registration messages.

:small_orange_diamond: 14. SIP Call Setup Steps

  1. INVITE – Initiates the call
  2. 100 Trying – Acknowledgment from the network
  3. 180 Ringing – The called party’s device is ringing
  4. 200 OK – Call accepted
  5. RTP Stream – Voice data exchange begins
  6. BYE – Call termination

:small_orange_diamond: 15. SIP 180 vs SIP 183

  • 180 Ringing: The called device is ringing.
  • 183 Session Progress: Early media (tones or announcements) before full call setup.

:key: Key Concepts

:small_orange_diamond: 16. Why IMS Is Required

IMS (IP Multimedia Subsystem) manages signaling, registration, and SIP to enable voice, video, and messaging services over IP networks.

:small_orange_diamond: 17. What Causes VoLTE Setup Delay

  • SIP signaling delays
  • DNS resolution issues
  • Bearer setup failure
  • IMS core congestion or faults

:small_orange_diamond: 18. Key KPIs for VoLTE

KPI Description
Call Setup Success Rate (CSSR) % of successful VoLTE call attempts
Call Drop Rate (CDR) % of calls dropped unexpectedly
SRVCC Success Rate % of successful VoLTE-to-CS handovers
SIP Success Rate % of successful SIP transactions
RTP Packet Loss % of lost RTP packets
MOS (Mean Opinion Score) Voice quality rating (1–5)

:small_orange_diamond: 19. What Is MOS

MOS (Mean Opinion Score): Subjective measure of perceived voice quality, rated from 1 (bad) to 5 (excellent).

:small_orange_diamond: 20. What Bearer Is Used in VoLTE

Dedicated EPS Bearer with QCI 1 – Low latency and high priority bearer dedicated to real-time voice.

:small_orange_diamond: 21. QCI 1 vs QCI 9

QCI Usage Characteristics
QCI 1 Voice Low latency, high priority
QCI 9 Internet/Data Higher latency, lower priority