Circuit Switched Fallback (CSFB)
Circuit switched fallback is the method by which a 4G phone without VoLTE drops back to a 2G or 3G network to make or receive a voice call.
What is Circuit Switched Fallback (CSFB)?
The phone is registered on LTE for data, and when a call arrives the network moves it to the older network, which adds call setup delay. As 2G and 3G are switched off, CSFB stops working and VoLTE becomes essential.