What is Carrier Unlock?
Carrier locks come with just about any cell phone you buy from a wireless operator. 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.
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.
This is not the case with phones developed for CDMA networks. This is the technology used by Verizon Wireless and Sprint in the U.S. It's also used by some carriers in Latin America, Asia, and Canada. The standard is not as widely used as GSM. CDMA devices do not have SIM cards. So if you wanted to take your CDMA device to another CDMA carrier, because a CDMA-only device can't be used on a GSM network and vice versa, you'd have to get the carrier to provision the device for that other network. EV-DO is the 3G technology used on CDMA networks.
In general, CDMA carriers don't reprovision devices made for other networks for use on their own networks. So this means phones made for Verizon, which are using the same basic technology as devices made for Sprint, won't work on Sprint's network. In other words, you can't reuse a Verizon device on Sprint and vice versa.
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.
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.