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 4your 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 anextra 50 msgs memory space.Switch phone off and back on.