The Yealink EXP40 expansion module is designed to be used with the Yealink T4 Series of IP desk phones, expanding the functional capability of your sip phone to a whole new level. It features a large graphic LCD, two pages of 20 flexible buttons are shown on the display can be programmed up to 40 various features, the productivityenhancing features include BLF/BLA, speed dialing, call forward, transfer, park, pickup, etc.