The GXP-2000 Extension Unit is designed to allow up to two units to be daisy-chained off each phone.
Each unit provides 56 speed-dial / BLF lines and can be powered either off the EXT. cable or using another power supply unit identical to the one that ships with the GXP-2000.
A small switch on the back of the Extension Unit is used to choose whether the unit is the first or second in a daisy-chain.