I built a cellular phone. It has a touchpad from old laptop and small LCD display. GSM module is SIM800L. Heart of a phone is Arduino Nano. Next I’am going to write a program to it so I can make and receive calls. Touchpad is very unique user interface. Case is made from plastic from old laptop.
Touchpad uses PS2 protocol. GSM modem uses softwareSerial library and LCD display is HD44780 compatible.
There is a place for SIM card in side of a phone and micro USB connector for normal phone charger. Back door is easily removable. It is fixed with magnet.