Cisco SIP IP Phone User Manual

B-54
Cisco SIP IP Phone Administrator Guide
AppendixB SIP Call Flows
Call Flow Scenarios for Failed Calls
Figure B-19 Cisco SIP IP Phone-to-Cisco SIP IP Phone—Authentication Error
Call from a Cisco SIP IP Phone to a Gateway Acting as a Backup Proxy
Figure B-20 illustrates a successful call from a Cisco SIP IP phone to a gateway acting as a backup
proxy.
Step Action Description
1. INVITE—Cisco SIP IP phone A
to SIP proxy server
Cisco SIP IP phone A sends a SIP INVITE request to the SIP proxy server. The
INVITE request is an invitation to User B to participate in a call session.
In the INVITE request:
• The phone number of User B is inserted in the Request-URI field in the form
of a SIP URL. The SIP URL identifies the address of User B and takes a form
similar to an e-mail address (user@host, where user is the telephone number
and host is either a domain name or a numeric network address). For
example, the Request-URI field in the INVITE request to User B appears as
“INVITE sip:555-0002@companyb.com; user=phone.” The “user=phone”
parameter distinquishes that the Request-URI address is a telephone number
rather than a username.
• Cisco SIP IP phone A is identified as the call session initiator in the From
field.
• A unique numeric identifier is assigned to the call and is inserted in the
Call-ID field.
• The transaction number within a single call leg is identified in the CSeq
field.
• The media capability User A is ready to receive is specified.
2. 407 Authentication Error—SIP
proxy server to Cisco SIP IP
phone A
SIP proxy server sends a SIP 407 Authentication Error response to Cisco SIP IP
phone A.
3. ACK—Cisco SIP IP phone A to
SIP proxy server
Cisco SIP IP phone A sends a SIP ACK to the SIP proxy server acknowledging
the 407 error message.
4. Resend INVITE—Cisco SIP IP
phone A to SIP proxy server
Cisco SIP IP phone A resends a SIP INVITE to the SIP proxy server with
authentication credentials.
Proxy
Server
IP IP
2. 407 Authentication Error
4. Resend INVITE B
3. ACK
1. INVITE B
IP Network
SIP IP
Phone User A
SIP IP
Phone User B
41477

Cisco SIP IP Phone Specifications

General

Brand	Cisco
Model	SIP IP Phone
Category	IP Phone
Language	English