Interrogating Call State Control Function (I-CSCF) performs the following functions:
is the contact point within an operator’s network for all connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator’s service area. It can be regarded as a kind of firewall between the external IMSS and the operator’s internal IMSS network. There may be multiple I-CSCFs within an operator’s network.
assigns a S-CSCF to a user performing SIP registration,
routes a SIP request received from another network towards the S-CSCF,
obtains from HSS the Address of the S-CSCF,
charging and resource utilization,
in performing the above functions the operator may use I-CSCF to hide the configuration, capacity, and topology of the its network from the outside,
additional functions related to inter-operator security are for further study.