IMEI number:
*#06#
Code to lock keys. Press together *7
Note: [] (pause) means the * key held in until box appears.

Select phone line - (use this to write things below the provider name):
[] [] [] 0 0 8 [] 1 []
Add phonebook to main menu:
[] [] [] 1 0 5 [] 1 []
Add messages to main menu:
[] [] [] 1 0 7 [] 1 []
Copy SIM memory (phonebook menu):
[] [] [] 1 0 8 [] 1 []
Eng Field options (main menu):
[] [] [] 1 1 3 [] 1 []
Slow (Frequency of search menu):
[] [] [] 1 0 1 [] 1 []
Medium (Frequency of search menu):
[] [] [] 1 0 2 [] 1 []
Fast (Frequency of search menu):
[] [] [] 1 0 3 [] 1 []
Enable EFR:
[] [] [] 1 1 9 [] 1 []
Function :
[] [] [] # # # [] 1 []

Change pin:
[] [] [] 0 0 4 [] 1 []
Unblocking using the "puk" number:
[] [] [] 0 0 5 [] 1 []

There are lots of similar codes exist. If you change the last number to 0 you can deactive that code. The 3 digit number at the middle are from 0 to 999. I put the most interesting codes. (EFR):Enhanced Full Rate Codec.

You can change GSM frequencies to 900/1800 by entering the enginnering model. Following the below steps:
enter menu and press 048263* quickly, then you will enter the secret engineering menu
under "Opcode"
input 10*0*3 for GSM 900
10*0*4 for GSM 1800
10*0*5 for GSM 1900
10*0*6 for dual band GSM 900/1800
10*0*7 for dual band GSM 850/1900

To add extra message space 4 your Motorola C350 C450 C550 or C650, press menu button, press 048263* quickly, then on the popup menu enter 47 press ok.press 50 and ok.press 1 ok.press 64 ok.press 1 ok.press 186 and ok.You will receive an extra 50 msgs memory space.Switch phone off and back on.(not tested)
Important!
Try these codes at your own risk! We don't accept any complaints.
Some of these codes may not work.