Device should support both legacy networks ( 3G/2G) and LTE ( VoLTE) for SRVCC or CSFB. That means while initial attach device would have requested with attach type2 which means both Combined/IMSI attach.
Suppose MME is getting CS call notification for MSC then How MME will notify to UE when 1. UE is idle mode. ? 2. UE is in connected mode?
How Circuit Switch Fallback works?