The EnGenius DuraFon PRO-BU is the expansion base that enables connection of an additional four (4) lines/ports to an existing DuraFon PRO installation. Also use the DuraFon PRO-BU for "Extended Range" options that are available with the DuraFon PRO product line. Multi-Line Functionality Supports up to 90 DuraFon PRO or DuraWalkie Handsets Works Behind any Analog Port (PBX, VoIP, PSTN, FXS or POTS) Expandable for Extended User Capacity