Snom ONE IP Technical Manual page 185

Deploying the snom ONE IP Telephone System
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cease ringing and then respond to the INVITE with a specific error response (a 487, as
shown below).
UAC
CANCEL
1
ACK
4
Figure B-4. CANCEL Request—INVITE Response 487 Request Terminated
Call Flow
1. A user makes a call, but after a few rings decides to hang up, so a CANCEL
request for the previous INVITE is sent by the client to the server.
2. The called party's phone stops ringing, and a 200 OK response for the CAN-
CEL is sent by the server to the client. The server then sends back a 487 Trans-
action Terminated response.
3. The client acknowledges the response by sending an ACK.
CANCEL sip:50@localhost;user=phone SIP/2.0
Via: SIP/2.0/UDP 192.168.0.32:2048;branch=z9hG4bK-
lg7xoa360cuh;rport
From: "PC PBX 51" <sip:51@localhost>;tag=u58kfgll03
To: <sip:50@localhost;user=phone>
Call-ID: 3c2c575f0429-38s24k0wrdrx
CSeq: 2 CANCEL
Max-Forwards: 70
Reason: SIP;cause=487;text="Request terminated by user"
Content-Length: 0
200 OK
487 Request Terminated
UAS
2
3