The CS FallBack (CSFB)  in EPS enables the provisioning of voice and other CS-domain services (e.g. CS UDI video, LCS, USSD) by reuse of CS infrastructure when the UE is served by E-UTRAN. A CSFB enabled terminal, connected to E-UTRAN may use GERAN or UTRAN to connect to the CS-domain. This function is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.

For more details download Leliwa Technical Bulletin