Will give you just an overview. In VoLTE roaming scenario, P-CSCF is of visited network and S-CSCF will always be of home network0 like PGW.
The SIP REGISTER request shall include a header P-Visited-network PLMN-id. This parameter is used to check if there is a roaming agreement between home network and the network represented by that PLMN-id.