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)