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Practical VoIP Using VOCAL by Luan Dang, Cullen Jennings, David Kelly

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Forking

In more complex implementations, SIP Proxies can be programmed to fork INVITEs to more than one destination. This is useful for people who are mobile or who use multiple phones in their work. For example, someone may have a phone in a lab and another in an office, on a different floor or in a different building, and may prefer to have both phones ring simultaneously for every call. This is known as parallel forking . This permits her to move freely from one work environment to another without having to remember to call-forward one phone to the other every time she changes location.

In other situations, the user might prefer to use a hunting method, also known as sequential forking, for receiving calls on multiple phone sets. Here’s how this works: All calls are first sent to a primary phone, maybe in an office. If that phone is not picked up within a configured number of rings, then the call is sent to a secondary phone, maybe a cellular phone. Eventually, if the call is not answered, it is usually sent to voice mail.

A Forking Example

The example shown in Figures 7-8 through 7-11 illustrates a parallel search for a called party.

The first 10 messages shown in Figure 7-8 are the same as the first 10 that are discussed under Figure 7-4. The 11th and 12th message, the INVITE sent by Proxy B to both phone B and phone C, are where the forking occurs. These messages are sent simultaneously to both phones, although they may not be received simultaneously.

In Figure 7-9, both ...

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