The ESI 55 5000-0737 is a VoIP phone designed for the ESI CS 50 and 50L, CS100, CS200, CS600, CS1000, and IP 900 servers.  The ESI IP 55, part number 5000-0737, has 30 programmable buttons, a backlit display, full duplex speakerphone, and headset jack.  The ESI 55 is the upgraded version of the IPFP model phones.