Configuring Advanced Features533BLF SubscriptionIP phones support BLF using a SUBSCRIBE/NOTIFY mechanism as specified in RFC 3265. Thisfeature depends on support from a SIP server.When the IP phone is configured to monitor a specific user, it sends a SUBSCRIBE message tothe server. A NOTIFY message which includes XML in the message body is sent to the IP phoneto inform the current state of monitored user. Once status of the monitored user is changedfrom idle to busy or vice versa, the IP phone is notified from the server with a NOTIFY message.You can manually configure the period of the BLF subscription.Example of a SUBSCRIBE message:SUBSCRIBE sip:1011@10.3.20.2:5060 SIP/2.0Via: SIP/2.0/UDP 10.3.20.1:5060;branch=z9hG4bK2940676338From: "1010" 10.2.1.48:5060>;tag=2493044525To: 10.2.1.48:5060>;tag=2527548726Call-ID: 0_3538292381@10.3.20.1CSeq: 2 SUBSCRIBEContact: Accept: application/dialog-info+xmlMax-Forwards: 70User-Agent: Yealink SIP-T46G 28.82.0.20Expires: 30Event: dialogContent-Length: 0Example of a NOTIFY message (confirmed shows the call has been established):NOTIFY sip:1010@10.3.20.1:5060 SIP/2.0Via: SIP/2.0/UDP 10.3.20.2:5060;branch=z9hG4bK276311022From: ;tag=3436332841To: "1010" ;tag=3098567568Call-ID: 0_4117916748@10.3.20.1CSeq: 4 NOTIFYContact: Content-Type: application/dialog-info+xmlMax-Forwards: 70User-Agent: Yealink SIP-T27P 45.80.0.20Subscription-State: active;expires=17Event: dialogContent-Length: 534-info xmlns="urn:ietf:params:xml:ns:dialog-info" version="3" state="partial"entity="sip:1011@10.2.1.48:5060">
