AppendixB SIP Call Flows
Call Flow Scenarios for Successful Calls
B-46
Cisco SIP IP Phone 7960 Administrator Guide
78-10497-02
Step Action Description
1INVITE—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 email 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 user name.
•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.
2INVITE—SIP proxy server to
SIP redirect server SIP proxy server sends the SIP INVITE request to the SIP
redirect server.
3300 Multiple Choices—SIP
redirect server to SIP proxy
server
SIP redirect server sends a SIP 300 Multiple choices
message to the SIP proxy server. The message indicates
that User B can be reached either at SIP phone B or Cisco
SIP IP phone C.
4INVITE—SIP proxy server to
Cisco SIP IP phone B SIP proxy server sends a SIP INVITE request to Cisco SIP
IP phone B. The INVITE request is an invitation to User B
to participate in a call session.