what is csfb
Circuit Switched FallBack  (CSFB) Circuit Switched FallBack. (CSFB) Circuit Switched FallBack (CSFB) is a technology whereby voice and SMS services are delivered to LTE devices through the use of GSM or another circuit-switched network. Circuit Switched FallBack is needed because LTE is a packet-based all-IP network that cannot support circuit-switched calls.