NOTIFY sip:aaaa@aaaaaaa;intercom=true SIP/2.0

....

Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY
Supported: timer, path, replaces
Event: talk
Allow-Events: talk, hold, conference, refer
Subscription-State: terminated;reason=noresource
Content-Length: 0