SIM locks come with just about any cell phone you buy from a wireless network. Even if you aren’t buying the device directly from a particular operator but are buying it from a retailer, such as Amazon or Best Buy, and it’s for a specific carrier, then most likely it has a carrier lock on it. This is regardless of whether you buy the phone with a subsidy and a two-year contract or if you buy the device at full price.

Generally, the only devices that do not have carrier locks are ones that specifically say they are “unlocked,” such as the Google Android Nexus brand of devices, the unlocked version of the iPhone, which is sold at the Apple store, and some developer edition devices. If you want an unlocked phone, you should research in advance whether the phone you are purchasing is available as unlocked and where you can buy it as an unlocked device. Remember: Paying full price alone doesn’t guarantee that a smartphone is unlocked out of the box.

What is a phone lock?
The lock is really a software code that’s put on the phone by the manufacturer as per the requirement of the carrier that sells the device. And the lock is meant to ensure that the phone can’t be used on any other operator’s network until a different software code is entered to unlock the device.

This is an issue that’s most important for devices that operate on GSM networks. This is a wireless standard that used by AT&T and T-Mobile in the U.S. and by most operators around the world, especially in Europe, Asia, Canada, and Africa, as well as parts of Latin America. The 3G technologies HSPA and HSPA+ are based on GSM, which means carriers offering HSPA or HSPA+ also operate via GSM.

All GSM devices are designed so that service is provisioned using a SIM card. With an unlocked device, a GSM smartphone can be reprovisioned and used on another network simply by popping out the old SIM card and putting in a new one from the new carrier. The carrier doesn’t necessarily need to be notified, and you don’t need anyone in the store to reprovision your phone.

But unlocked GSM phones from AT&T and T-Mobile will work on each other’s networks, which makes the use of GSM much more flexible than CDMA.

Why unlock my phone? In summary, phone locks are not really relevant when you’re talking about 2G and 3G devices that operate on CDMA or EV-DO networks. But phone locks are very important for devices that operate on a GSM or HSPA/HSPA+ network. Almost every GSM device comes “prelocked” to a particular carrier. Certain phones are sold unlocked. And if you have a device that is locked, you can get it unlocked from your wireless carrier if you meet certain criteria, which includes paying the full price of your device or ending your contract and being in good standing with your service provider.