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.
Coolwave Communications BV is a Netherlands-based telecom operator serving the rapidly growing market for voice,voice-mobile, VOIP, internet, data and fiber services.