CSFB (Circuit-Switched Fallback)
Updated
CS Fallback is the mechanism by which a 4G device without VoLTE moves to 2G or 3G to make or receive a call, then returns to 4G. It is specified in 3GPP TS 23.272.
In practice
CSFB depends on 2G or 3G. When they close, a device that relies on it can no longer make calls, even if its 4G data still works. SMS can often still travel over 4G signalling, depending on the network.