SIP method unsubscribe

At some point, either the Client’s representative User Agent Server or the Gateway may decide to terminate the monitoring session. This is achieved by sending an UNSUBSCRIBE request to the correspondent server. Such a request indicates that the sender intends to close the monitoring session immediately, and, on receipt of the final response from the receiving server, the session is deemed over.

Note that unlike the SUBSCRIBE request, which is never sent by a PINT gateway, an UNSUBSCRIBE request can be sent by a PINT gateway to the User Agent Server to indicate that the monitoring session is closed.
(This is analogous to the fact that a gateway never sends an INVITE, although it can send a BYE to indicate that a telephone call has ended.)