As per this link VoLTE got launched in South Korea last year. But it is not clear as to what technology(CSFB or SRVCC) they are using.
Network operators are usually very finicky to upgrade their infrastructure (which is a must for SRVCC), therefore they prefer CSFB(no upgrades required to infrastructure),despite its limitations.
CSFB is a 3GPP-defined standard solution that requires terminals be equipped with either dual-mode/single-standby or dual-mode/dual-standby capabilities. In this case the UE controls the handover of the voice bearer between PS and CS. When UE in IMS call detects waning LTE coverage, it setsup a call with on the CS(and tearing down the PS leg), lands back onto IMS when it reacquires LTE coverage by setting up the PS call(and tearing down the CS leg)This is not a very reliable solution as UEs cannot be trusted to be the control points. Also since the UEs are mandated to have dual modes it is battery intensive.
SRVCC is supposed to continue, an IMS call on LTE network, seamlessly over a 3G/2G network, major requirement being, NOT to have the UE connect to 2 different RAT’s simultaneously thus implying more battery life. There are some upgrades required on the MME/SGSN/MSC. EUTRAN has to detect GERAN and UTRAN. HSS needs to support SRVCC relevant flags. PCRF needs to enforce bearer over QCI 1(dedicated voice bearer). UE has to be IMS compatible and needs to send SRVCC IEs during attach procedure.
When the IMS/LTE call is active and if EUTRAN detects that UE is moving towards a UTRAN/GERAN, it will initiate a handover procedure for UE towards the SGSN. A PS handover will be initiated for all bearers except for the one with QCI=1. MME will then initiate a PS-CS handover for QCI =1 bearer towards MSC (over Sv interface b/w MME to MSC). MSC setsup the call with the UE and continues the voice, whereas IMS will now send re-INVITEs to the other call leg to update their SDPs(?)