What is SRVCC ?

It is Single Radio Voice Call Continuity.  This capability provides handover from a voice call over LTE based packet network to the circuit based network.  The LTE uses IMS for a voice call over data network. This does not involve of setting up a dedicated E1 or T1 channels for voice calls.  During a call this may happen , that a subscriber moves to the network where only 3g or 2g network is available.  SRVCC provides uninterrupted call experience from VoLTE to 3G or 2G.

While registration to the home network, the MME send the SRVCC capability in Update Location Request (ULR) message.