Call Transfer
One useful feature of IP telephony is the fact that users can easily transfer their calls to someone else.
Imagine for example that you are talking to a friend who’s asking you about your notes of a particular class or lecture. You don’t have them but you know that another friend of yours does. It would be nice if you could simply forward the call to that new person so that your two friends could arrange the transfer by themselves.
We would like to implement that kind of functionality in SIP Communicator using the SIP protocol. With SIP (and with most other protocols for that matter) one can transfer a call in two different ways. You could either blindly forward the call to a number without knowing whether someone would actually answer, or you could first establish the connection, ask the corresponding person whether they are interested in taking the call and only then execute the transfer.
These two types of transfer are often referred to as attended and unattended (or blind) transfer. They involve the exchange of a number of specific messages.
Here’s how one could perform a blind transfer with SIP:
The following image shows the messages exchanged during an attended call transfer.
The mechanisms for transferring calls are described in detail in RFC 3515 and RFC 3891.
References:
The Session Initiation Protocol (SIP) Refer Method
http://tools.ietf.org/html/rfc3515
The Session Initiation Protocol (SIP) “Replaces” Header
http://tools.ietf.org/html/rfc3891
Other SIP Communicator student projects
http://www.sip-communicator.org/students
The official SIP Communicator website
http://www.sip-communicator.org
The SIP Communicator developer website on Java.net
http://sip-communicator.dev.java.net/index-jn.html
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