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.
Abbeynet is a leading European company focused on state-of-the-art VoIP technologies based on the SIP standard, including integration between fixed and mobile networks in an all-IP context and web-based VoIP service architectures.