Yealink SIP-T2 SERIES Administrator's Manual page 340

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
Call completion allows users to monitor the busy party and establish a call when the
busy party becomes available to receive a call. Two factors commonly prevent a call
from connecting successfully:
Callee does not answer

Callee actively rejects the incoming call before answering

IP phones support call completion using the SUBSCRIBE/NOTIFY method, which is
specified in draft-poetzl-sipping-call-completion-00, to subscribe to the busy party and
receive notifications of their status changes.
The caller subscribes for update notifications of the dialog event from the busy party.
Example of a SUBSCRIBE message:
SUBSCRIBE sip:1000@10.10.20.34:5060 SIP/2.0
Via: SIP/2.0/UDP 10.10.20.32:5060;branch=z9hG4bK2880274891
From: "10111" <sip:10111@10.2.1.48:5060>;tag=8643512
To: <sip:1000@10.2.1.48:5060>;tag=4025601441
Call-ID: 4_2103527761@10.10.20.32
CSeq: 2 SUBSCRIBE
Contact: <sip:10111@10.10.20.32:5060>
Accept: application/dialog-info+xml
Max-Forwards: 70
User-Agent: Yealink SIP-T23G 44.80.0.60
Expires: 60
Event: dialog
Content-Length: 0
Example of a NOTIFY message (The subscription (SUBSCRIBE message) of the dialog
event ―Call Completion‖ is confirmed by the busy party):
NOTIFY sip:10111@10.10.20.32:5060 SIP/2.0
Via: SIP/2.0/UDP 10.10.20.31:5060;branch=z9hG4bK1830418099
From: <sip:1000@10.2.1.48:5060>;tag=1032948194
To: "10111" <sip:10111@10.2.1.48:5060>;tag=722495580
Call-ID: 0_160090766@10.10.20.32
CSeq: 2 NOTIFY
Contact: <sip:1000@10.10.20.31:5060>
Content-Type: application/dialog-info+xml
Max-Forwards: 70
User-Agent: Yealink SIP-T23G 44.80.0.60
Subscription-State: active;expires=60
Event: dialog
Content-Length:
318
584