As S-CSCF1 knows that the next hop I-CSCF is in the same home network (and therefore, a loose
router), it includes a Route header.
In the case where the Request-URI of the incoming INVITE request to S-CSCF contains a
TEL-URL, it has to be translated to a globally routable SIP-URL before applying it as
Request-URI of the outgoing INVITE request. For this address translation the S-CSCF uses
the services of an ENUM-DNS protocol according to RFC 2916, or any other suitable
S-CSCF1 adds the identifier of its own
network to the originating Inter Operator
Identifier (IOI) parameter of this header.
I-CSCF2 sends a query to the HSS to find out the S-CSCF of the called user. The HSS responds with
the address of the current S-CSCF for the terminating subscriber.