Circuit Switched Fallback (CSFB), with the help of a global system for mobile communication (GSM) or another circuit-switched network, delivers voice and SMS services to LTE devices. The necessity for a Circuit Switch FallBack for the packet-based all-IP network, LTE, which cannot support circuit-switched calls. For instance, when an LTE network is not available to make or receive either a call or SMS message, the device "falls back" to a more accessible 3G or 2G network to finish the call or to send the SMS message. CSFB is often viewed as a temporary solution.
The solution that is expected to deliver voice services over the LTE network in the long-term is Voice over LTE (VoLTE).Download Dummies Guide