Main functions: 1) Uninterrupted call: the mainframe is able to display calls from multiple extensions, and memorize them. Extensions can perform incoming calls in all status, such as standby, talking and broadcasting 2) Handheld call intercom with a handheld caller, the extension can realize call, call release, intercom and LED display 3) Duplex intercom is available between the mainframe. LED channel of mainframe can choose indicator light flash, and music is on promptly 4) Group call function. The mainframe can realize calls to intercom with partial extensions at the same time 5) Volume set up 6) Corridor display 7) Many kinds of extensions are optional according to your need