Some telephones, depending on the brand and model, have the intercom function, which allows to begin communication by sending the signal. When the signal comes, the telephone set rings a signal different from the standard ring tone call. After that the telephone temporarily switches off from the telephone line and works as an intercom.
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.