Interrogating Call Session Control Function (I-CSCF) is a key element in the IMS roaming methodology. It enables requests to be routed to the correct Serving Call State Control Function. As there may be several S-CSCFs either within a network, or if a roaming user requests access.
The I-CSCF interrogates the HSS to obtain the address of the relevant S-CSCF to process the SIP initiation request.