S-CSCF1 first authorizes the subscription. As S-CSCF1 can trust the content of the P-Asserted-Identity
header and <sip:firstname.lastname@example.org> is on the list of the authorized users for the "reg"
event package stored by S-CSCF1, therefore S-CSCF1 sends an acknowledgement towards the UE
indicating that the subscription was successful. This response will traverse the path that the
SUBSCRIBE request took as described in the Via list.