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A Private Branch Exchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. 

PBXs make connections among the internal telephones of a private organization—usually a business—and also connect them to the public switched telephone network (PSTN) via trunk lines.

Trunk

A trunk line is a circuit connecting telephone switchboards (or other switching equipment), as distinguished from local loop circuit which extends from telephone exchange switching equipment to individual telephones or information origination/termination equipment.

When dealing with a Private Branch Exchange (PBX), trunk lines are the phone lines coming into the PBX from the PBX from the telephone provider. This  This differentiates these incoming lines from extension lines that connect the PBX to the PBX to (usually) individual phone sets. 

Extensions 

A telephone extension is an internal telephone line attached to a Private branch exchange (PBX). The PBX operates much as a community switchboard does for a geographic telephone numbering plan and allows multiple lines inside the office to connect without each phone requiring a separate outside line. In these systems, a dialer usually has to dial a number to tell the PBX to connect with a landline to dial an external number. Within the PBX, the user merely dials the extension number of the person. Each phone line may be extended up to a fixed maximum.

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Secure Call

A Secure CallA secure call is a voice connection which can't be wiretapped and it runs over Voice Over IP (VoIP) communication protocol.

End to Site security model

Secure call is encrypted from client up to server.
The end to site security model provides a strong security level and can be used among two or more

• 3-way calls
• call transfer
• conference rooms

End to End security model

Secure call is encrypted from client up to the other client. Despite server relays encrypted traffic, it does not knows the encryption keys, so it cannot decrypt the call content.
The end to end security model provides the highest security level but can be used only between two

In this security model the server cannot decrypt secure call content, so advanced features are not available. 

Multilevel security model

This is a mix of the other security model. Each call made by

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client produces encrypted traffic based on the recipient of the secure call:

• if the call recipient is another

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• Brand
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  client, the call is made using the ZRTP protocol for key exchange
• if the recipient of the call is not a

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• Brand
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  client
• the call is made using the SDES protocol

This security model can be used to integrate enterprise PBXs.

It is also possible to provides advanced telephony features such as:

• 3-way calls
• call transfer
• conference rooms

using SDES protocol.

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Brand
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the VoIP client for Secure Calls connections. It has to be used along with

Conference Rooms

The Conference Room is the kind of call that more persons can partecipate. The conference calls are usually defined as "rooms", whose access can be limited by time settings or pass code. 

Conference Calls

Conference Call, as its name implies, is a call involving at least three users. It differs from Conference Room by the fact that Conference Call is one Secure Call at which third parties got invited. So it's one sort of dynamic conference room. All users invited are thus added to conversation in progress by either caller or callee.

Call transfer

Call transfer is a typical PBX performance which is implemented in  PrivateServer  as well. One of the partecipant can hold on his/hers peer and perform a new call to the number to which trasfer the call. If the desired number picks up the call, then the transferrer can close the communication and let his/hers peer talk with the trasferred number.

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