The traditional conference calls are made using analogue telephones. It is just the same way that audio conferencing over VoIP works. What exactly happens in a conference call is that, there is a server to which the callers get connected. It is called the conference bridge. The same enables talking among multiple people possible. This is the same principle on which audio conferencing over VoIP works. Here, the callers are connected to the server through computers or telephones.
Both the traditional conference calling and VoIP conference calling are different from each other. Major difference is regarding the movement of voice data. Circuit switching method is adopted by the traditional telephones. In this method, there are a series of switches that are interconnected, through which the calls are routed by the telephone system, till they arrive at the server that enables multiple people to make calls to each other. In offices, there would be a small phone network called the PBX (Private Branch Exchange). This is the network used for connecting the callers to the conference bridge, if all of them happen to be within one office building. Until the call lasts, the connection will be opened and most of the times, only certain portions of connection are in use, since normally one person will be talking at a time.
The method that VoIP networks use is that of packet switching. This method keeps the connection open only for as long as you can send a small packet size data between two systems. Here, the data travels over the internet and they usually follow the path which is the most efficient.
VoIP networks, most often, have in built audio conference calling facilities. At times the same may be available as an upgrade in service. These VoIP networks, when used in business, outside company calls can also get connected to the connection bridges. Either the lines that are external get connected to the server directly or through VoIP network, such external calls may arrive at the bridge server.
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