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.
MCK communication is a part of Citel technologies, The VoIP Migration Company. MCK products like EXTenders, Gateways, and DTI call center solutions, are now available through Citel.