IMS R3-3a. SIP NOTIFY request: from S-CSCF1 to P-CSCF1

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IMS R3 signalling flow - UE subscription for registration state event package - IMS R3-3a. SIP NOTIFY request: from S-CSCF1 to P-CSCF1

From: The tag of this field matches that of the "To" field in the received 200 (OK) response for the SUBSCRIBE request.
Content-Type: Set to the value of the Accept header received in the SUBSCRIBE request or "application/reginfo+xml" if the Accept header was not present in the SUBSCRIBE request.