From RFC 2848:
During the subscription period, the Gateway may, from time to time, send a spontaneous NOTIFY request to the entity indicated in the Contact: header of the “opening” SUBSCRIBE request. Normally this will happen as a result of any change in the status of the service session for which the Requestor has subscribed.
The receiving user agent server MUST acknowledge this by returning a final response (normally a “200 OK”). In this version of the PINT extensions, the Gateway is not required to support redirects (3xx codes), and so may treat them as a failure.
Thus, if the response code class is above 2xx then this may be treated by the Gateway as a failure of the monitoring session, and in that situation it will immediately attempt to close the session (see next).
The NOTIFY request contains the modified session description. For example, the Gateway may be able to indicate a more accurate start or stop time.
December 2, 2021
Phone System 