Lte is a packet only network and handles packet only data traffic. But if UE is registered on LTE and service user make a CS call (circuit switched or voice call) or UE has to receive an MT CS call (mobile terminating call or incoming CS call) from the existing CS domain through EPC.
There are few options (VoIP, IMS, CSFB etc) to handle the CS call (Mobile Originating & Mobile Terminating) in LTE and one of them is CS fall back which uses existing technologies which support CS traffic like GSM, WCDMA to make voice call happen. CSFB makes UE to move to the CS supported RATs using redirect or handover and perform the CS call.
In-order to perform CSFB, UE should support multiple radio access (Lte & Utran or Geran) and EPC should support CSFB. When I say EPC should support CSFB, it means co-ordination should be there at the network end between Lte and other CS enabled RAT (Utran or Geran). It's not very simple to make these two RATs with different protocol stack, work together !! specially at the network end.
For more detail please please see the following link http://wirelessscan.blogspot.in/2013/02/CSFBL2W.html (explains only attach part)
