The P-CSCF adds itself to the Record-Route header and Via header. As the request is forwarded to an
interface that is not compressed, the own P-CSCF SIP URI does not contain the "comp=sigcomp"
The P-CSCF removes the Security-Verify header and associated "sec-agree" option-tags prior to
forwarding the request. As the Proxy-Require header is empty, it removes this header completely.
P-Asserted-Identity: P-CSCF inserts the TEL URI in
the P-Asserted-Identity header field and it also removes
P-Preferred-Identity header field.
P-Charging-Vector: The P-CSCF inserts this header and
populates the icid parameters with a globally unique value.