The ESI 55D is a digital phone designed for the ESI CS 50 and 50L, CS100, CS200, CS600, CS1000, and IP 900 servers. The ESI 55D, part number 5000-0736, has 30 programmable buttons, a backlit display, full duplex speakerphone, and headset jack. The ESI 55D is the upgraded version of the DFP model phones.