VoLTE : What are the roles for P-CSCF, I-CSCF and S-CSCF nodes during establishing voice call ?

Question

Is it possible HSS belongs to home network and S-CSCF belongs to visited network for the Cx interface ? If not then why ?

Answer 1

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.