A cellular phone that uses the Global System for Mobile (GSM) network and can access multiple cellular phone service provider networks is called an unlocked GSM phone. A locked cell phone is sold by the cellular phone service provider and cannot be used with any other service provider. Each service provider locks the phones they sell to restrict their transferability and to require consumers to purchase a new phone when switching service providers.

Unlocked GSM phones can be purchased from stores that are not affiliated with a specific cellular phone service provider. These phones typically cost more than the locked phones, as the cost of the phone is discounted by the service provider. The longer the service contract, the larger the discount available on the phone itself.

The GSM network is a digital signal network used in more than 212 countries and territories around the world. Although all GSM cell phone users can access the network to make and receive calls, they are charged additional access or roaming fees by their cellular service plan provider for activity using another companies network. Having an unlocked GSM phone allows customers to take advantage of the network as they travel the world and pay only the local cell phone usage fees.

Due to the digital format of the GSM network, the development of short message service (SMS), or text messaging, was possible. The worldwide emergency phone number 112 was also developed due to the expansion of the GSM network to allow international travelers to quickly locate emergency services when needed. Further enhancements to this network increase the strength of the signal in remote areas and further decreases the costs to the service providers.

Unlocking a GSM phone requires changes to the software settings on the phone to allow the use of a subscriber identity module (SIM) card from other service providers. The phone number of the cell phone and the information required to connect the phone to a cellular service providers network is stored on the SIM card. The card itself is the size of a post office stamp and is usually located under the battery of the cell phone.

You can purchase an unlocked GSM phone and then purchase a SIM card from the service provider you want to use. If you are traveling in the USA, Europe or Asia, it is possible to purchase the SIM cards directly from the cell phone service providers on a pay per use service plan. You may need to purchase multiple cards if you plan to travel widely, so that you are not charged roaming fees.

To access the cell phone services, take your unlocked GSM phone, insert the SIM card for the local service provider and turn on the phone. You are now able to make and receive phone calls and text messages using the SIM card service provider network. Take note of the phone number for the SIM card installed, as it will be displayed when you power on your phone.