Because the Samsung SGH C300 is a GSM-type phone, it is able to be unlocked for use with carriers alternative to the one it was purchased on. Before this can happen, however, some proactive action is required to get the process started.
Call your service provider to request the subsidy unlock code for your Samsung SGH C300. When a customer service representative comes on the line, you will be asked for a few pieces of information about your device. Using this data, an unlock request will be sent to Samsung and the code will be sent to you via e-mail within two weeks. Do not proceed until the code has been received.
Insert a SIM card from an alternate carrier into the device and power it up. After a longer-than-normal load time, a white screen will appear with an "Insert Correct SIM Card" error displayed.
Carefully enter the unlock code for your Samsung SGH C300. To do this, use the keypad to enter "#0111*," then key in the actual unlock code, followed by a "#." Press "OK." If the code was entered correctly, a confirmation message will pop up indicating that the device is now unlocked.
Before calling your carrier to request the unlock code, write down the IMEI number of the device (which can be found by typing "*#06#") so that you have it ready to give out upon request.
Samsung devices provide only five attempts for successful unlock. Wasting attempts by making guesses at the unlock code could permanently lock the phone to your current carrier. Do not attempt to unlock the phone until the code has been received.