This example assumes that UE1 and P-CSCF1 both have subscribed for the user's registration state event
package and shows how UE1 and P-CSCF1 are notified when the network-initiated
deregistration event occurs in S-CSCF1.
After S-CSCF1 deregistration notification procedure S-CSCF1 immediately sends a NOTIFY
request towards UE1 in order to inform about the network initiated deregistration and the subscription
termination. The same Request URI, To, From, Call-ID are used as in the first NOTIFY request. CSeq
is incremented since this is the second NOTIFY request sent towards UE1.