Update registration timer: As the REGISTER request arrived integrity protected, S-CSCF1 does not need to challenge the user, but just update the registration timer to the value requested by the user (if the policy of the network allows it).

NOTE: S-CSCF1 is allowed to challenge the user. If S-CSCF1 decides to challenge the user, the call flow will be similar to the one presented in "Registration: user not registered".