Tyler Spadgenske has written an Instructable that shows you how to create your own mobile phone using a Raspberry Pi A+. He 3D-prints a case for the phone and then fills that case with components including an Adafruit FONA board (which connects to the mobile network once you insert a SIM card). It’s a great project that was probably based on Dave Hunt’s PiPhone work. Read Tyler’s Instructable here. It contains all the information you need, including a link to his ‘operating system’ which is the GUI script.