The display shows up the battery status, Missed calls count and Unread SMS on 8 digits 7 Segment display module. The communication with the accessory is using Bluetooth and one Arduino board that interfaces with the display and the Bluetooth unit.
You need to run the program once (per boot of the phone), and even if the program is in the background, when the accessory and the phone are in range of Bluetooth communication, the display will show the status of your phone. Pretty much like Car hands free set, that automatically connects with your phone.
The code runs on Android phone and source code of the Arduino accessory and the Android phone code are shared here in step 4.
Updated code can be found in here:
Made several small enhancements and fixed an issue some where complaining about, where connection was not always made from phone to the device. The problem as I learned was in new Firmware of RN-42 that required a bit of configuration to work with Android. Those changes include using commands SI,0200 and SJ,0200 for the BT module.