GSM (Global System for Mobile) technology works by attaching account information to a SIM card.
To understand what a GSM-unlocked phone is and what it does, you need to understand what GSM technology is and how it works.
What is GSM technology?
GSM (Global System for Mobile) is a special kind of phone technology. It works by attaching account information (like your phone number, plan details, contact list, and more) to your SIM card.
When you put that SIM card into a compatible device, you can use that device to make calls, send text messages, and download apps. If the phone you're using is not GSM-compatible, it will not be able to access the information on the SIM card.
Locked vs. unlocked phones
Sometimes, companies that sell phones only want you to use the phone with their SIM cards, so they'll lock the phone. For example, if you buy a phone that is locked to the T-Mobile network, you will not be able to use it with a RedPocket SIM card. You can only use it with a T-Mobile SIM card until the phone is unlocked.
However, if you buy an unlocked phone, you can use it with any SIM card, including RedPocket's SIM cards. Buying a phone directly from the phone manufacturer instead of from a service provider is one way to get an unlocked device. For example, if you pay full price for an iPhone directly from Apple instead of buying it from a service provider like AT&T, you will get it unlocked.
You can also get unlocked phones from retailers like Amazon or eBay, or even physical stores like Fry's Electronics and Best Buy. Read the product description to make sure that "GSM Unlocked" is specified, or it won't work with your SIM card.