Circuit-Switch Fallback (CSFB) enables circuit-switched voice and SMS services to be delivered to Long Term Evolution (LTE) devices. When an LTE handset makes or receives voice calls, the device "falls back" to the 3G or 2G network to complete the call. This feature is needed because LTE is an all-IP, packet-based network that cannot support circuit-switched calls.
CSFB requires a new interface to be set up between the legacy mobile switching center (MSC) and the mobile management entity (MME) in the LTE core network. It was specified in 3rd Generation Partnership Project (3GPP) Release 8.
CSFB is often described as an interim solution for LTE operators, while Voice over LTE (VoLTE) is considered to be the longer-term goal for delivering voice and other media-rich services on LTE networks.