Based on the user's URI, P-CSCF1 determines that UE1 is registering from a visiting domain and
performs the DNS queries to locate the I-CSCF in the home network. The look up in the DNS is based
on the domain name specified in the Request URI. The DNS provides P-CSCF1 with the address of
the I-CSCF in the home network.
NOTE: The P-CSCF must not use the I-CSCF address cached as a result of
the previous registration.