this is a LOW cost $15 simple mobile phone that i created using arduino (homemade)and SIM900.due to the less number of pin .i use two microcontroller one for display and one for keypad and SIM900 are serially connected with each other.here you can call a person and receive a call from person

Step 1: Homemade Lcd Module

this a separated lcd module which is controlled using only one pin Tx . its like arduino with lcd module.here the work of the atmega328 only to display the information that are received .here is the tutorial https://www.arduino.cc/en/Tutorial/HelloWorld lcd tutorial

Step 2: Connecting Sim900 and 4*4 Keypad

i use 4*4 keypad .when you enter any value from keypad it can store in a array.And when press OK it send AT command to sim900 to make a call .then make delay in between delay send the LCD module to print the appropriate information on lcd . here is the tutorial http://playground.arduino.cc/Main/KeypadTutorial you should download the library for the keypad interface .you should also learn about the different AT command for sim900 http://www.electrodragon.com/w/SIM908_SIM900_Common_AT_Commandsat command,keypad tutorial

Step 3: Scope for Updating

mobile phone videoi have only add a small functions to make a call and receive a call.you can add different function like SMS sending receiving etc . watch the video for more information.hope it help you a lot .any question feel free to ask.thank you.here the video link https://youtu.be/Emia_ZSxdGs