SRVCC - Subscribers VoLTE call will drop if out of VoLTE coverage?

SRVCC - Single Radio Voice Call Continuity

    Does the VoLTE call drop when out of VoLTE enabled cell coverage? Answer is Yes and a No. Potential solution is supporting SRVCC for call continuity with inter-RAT systems. SRVCC is adapted as standard but not something developed for VoLTE. SRVCC was designed to facilitate IMS Achored voice call continuity between LTE/HSPA network and GSM/UMTS/CDMA Networks.  This not as simple solution, since the voice call continuity is between Packet switch network and Circuit switched networks.

    Attach procedure is used to exchange the information about letting each other know SRVCC capability of UE and MME.  MSC-S, MME and UE needs to support SRVCC for the voice call continuity. Series of handover procedures carried out to move the voice leg from one RAT to another.

VoLTE - Voice over LTE - Must Know!

How will UE know that VoLTE is supported by the EPC network?

    During the NAS Attach procedure or NAS TAU procedure, in the Attach Accept and/or RAU Accept message, additional IE called "EPS Network Feature" will be included indicating "IMS Over PS session" is available. This triggers the UE to register with IMS network.

VoLTE Call Procedures - Call flow:

  In VoLTE call establishment procedure is achieved using a application server called Telephony Application Server(TAS).
Procedure includes:

• Establishment of Default EPS Bearer with  QCI = 5
• SIP Call Establishment Procedures
• Establishment of Dedicated EPS Bearer to support Media Bearer
• SIP Call Termination Procedures
• Termination of Dedicated EPS Bearer