In the past, when you call an office building or business, you would either be transferred to the department you want to talk to, or given their extension number. This extension is usually a 2-3 digit code that when dialed, transfers you automatically to the particular person or department you want to speak with. The reason why this code works is because it is programmed into a PBX system.
nfon is an IP-Centrex provider for enterprise customers. As the German market leader, nfon offers its technology also as a white label solution to other providers.