CSFB refers to "Circuit-Switched Fallback". It is a mechanism used in case the 4G network is still not ready for handling voice calls of 4G connected users.

It allows the users to seamlessly transition from the 4G packet-switched network, to the 3G/2G circuit-switched network, in order to initiate or receive a voice call.

CSFB is a crucial feature in 4G networks, as it allows mobile devices to maintain access to voice and other legacy services when the 4G network is not suitable for those services.