GSM, or Global System for Mobile Communications, and CDMA, or Code Division Multiple Access, are the two major types of wireless telecommunication standards used by mobile devices around the world. Many mobile devices are designed to work on either the GSM network or the CDMA network. You cannot use a GSM SIM card in a mobile device that is designed to work exclusively with CDMA because CDMA and GSM are two different technologies. However, you can use a GSM SIM card in a mobile device which incorporates both CDMA and GSM technology. Check your phone's specifications to see if it has this capability.
Turn off the cell phone.
Release the phone's battery cover lock and lift the battery cover off the phone. Very often pressing down on a phone's battery cover, while at the same time pushing forward on the cover, will release the cover's locks. On some cell phone models, however, there may be a button, latch or switch that releases this lock.
Remove the battery from the phone. Locate the phone's SIM card compartment. The SIM card compartment is often located within a cell phone's battery compartment. It may be concealed by a small cover.
Insert a SIM card issued by a GSM network carrier (such as AT&T or T-Mobile) into the phone's SIM card compartment. The gold contacts located on the front side of the SIM card should face down when the SIM card is inserted.
Put the phone's battery back in. Place the battery cover back on the phone and lock it into place.
Turn on the cell phone.
Refer to the user's manual which came with your device to learn how to switch the phone's mode from CDMA to GSM in order to power up the phone in GSM mode. This process varies, depending on the brand and model of the cell phone you are using.
If your hybrid CDMA-GSM phone is locked to a specific GSM network carrier (T-Mobile, for instance), in order to use a GSM SIM card issued by an alternate GSM network carrier, the phone’s SIM lock has to be unlocked using the phone’s unique SIM unlock code. To calculate the phone’s SIM unlock code you need to know it 's unique IMEI or ESN identification number. This number is printed on a label stuck inside the phone’s battery compartment. Once you know this number you can purchase the SIM unlock code, along with instructions for how to enter the code in your particular model phone, from a SIM unlock code service for a small fee. Or you may also be able to obtain the code and accompanying instructions from the phone's current network carrier, at no charge, if the carrier offers unlock code services to its customers.