You can change motorola 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.