P-CSCF is the edge node of IMS network. I meant to say this is the first node to which an UE contacts (SIP registration).
Apart from involving in SIP registration, P-CSCF does the following jobs as well:
1- IP security association between UE and network.
2- Compression and decompression of SIP messages for the better utilization of available radio resources at eNodeB.
I-CSCF contacts HSS node to get the appropriate S-CSCF node of the network to forward the SIP request message.Sometime HSS provides a list of available S-CSCF nodes and I-CSCF selects one of them.
S-CSCF is the actual node which provides VoLTE services to its subscriber. This node also finds out the PGW address of second party of VoLTE call. I meant to say, receiver of the call.