This is only a  notice for myself or just a note, it’s be better to read Amazon Unlocked Phones Help Guide before you buy an unlocked phone from amazon. But, of course you should read all help guide from each online shop before you buy something from them. The purpose is to reduce the risk of selecting the goods or any misunderstandings between the seller and buyer, straight read from quote below :

What is an unlocked phone?
GSM cell phones use a SIM (Subscriber Identification Module) card that stores subscriber data. The SIM card is issued by a carrier and provides cell service by activating any phone into which it is inserted.
A locked phone will only recognize a SIM card from a particular carrier. The “lock” is a software setting that keeps the cell phone “loyal” to one carrier.
Unlocked devices are cell phones that recognize SIM cards from any carrier.

 

Why do Carriers lock phones?
A locked cell phone will not allow users to change phones at their leisure. A locked cell phone benefits only the carrier by keeping a customer using their service.
Some cellular carriers require that you purchase a locked cell phone for the contract period, but will unlock the phone at the time that the contract expires.

 

What are the benefits of unlocked phones?
Unlocked phones give consumers more control of plans, pricing, and services and make carriers work for you.
Unlocked phones give consumers the freedom to move your SIM card from device to device without signing another carrier contract. This means consumers can easily switch to a different phone whenever they want. This gives them more freedom if their device breaks or is damaged, or if they want to upgrade.
Some unlocked phones are sold directly by the manufacturer, not from the carrier. Sometimes carriers like to alter the phone to fit certain requirements. These locked phones can have alternative carrier software, or have missing pieces of the hardware, such as WiFi or GPS. Unlocked phones sold directly by the manufacturer are un-altered by the carrier, and don’t have any altered software or missing hardware.

 

Why are unlocked phones more expensive?
Most cellular carriers require that customers sign a contract when they purchase a locked cell phone. Carriers subsidize the price of the phone to make an attractive offer, but require the customer to sign a contract so the lost revenue can be made up over the time period of the contract. However, if the phone is ever broken or lost, the customer must buy another phone, which the carrier usually sells for full-price. Unlocked phones are not subsidized, so they must be sold at full-price, yet the benefit is that the customer does not need to sign a contract or contract extension.

 

What are the international benefits of unlocked phones?
International roaming charges can be quite expensive. Sometimes customer pay a few dollars a minute to make phone calls in foreign countries. Unlocked phones give customers the freedom to travel without paying for roaming. Customers with an unlocked phone can purchase a pre-paid SIM card in the foreign country they are visiting, and pay local rates for local phone calls. To call internationally, the customer can purchase an international phone card with a local access number; thus, the customer is only charged the rate for a local phone call, plus the price of the phone card.
Another alternative is to use the phone as a VoIP device. With VoIP (Voice over IP), the phone makes phone calls over an Wi-Fi spot with VoIP services such as Skype. There may still be a charge, but it is minimal compared to international roaming charges.

 

More Questions

Can I use an unlocked phone?
Unlocked phones can be used on any GSM network world-wide. Most networks in the world-wide operate on a GSM network. In the United States, the largest GSM networks are AT&T and T-Mobile. Unlocked cell phones will not work on CDMA networks, such as Verizon Wireless and Sprint.

I just bought an unlocked phone; how do I activate it?
Switching to your new phone is easy. Remove the SIM card from your current cell phone (usually located near the battery) and insert it into the new phone. Turn the new phone on (make sure the phone is charged). Your new phone will now send and receive phone calls with your original phone number. There is no reason to call you carrier.

I am in the middle of my carrier contract but I want to upgrade my phone.
With an unlocked device you can purchase a new phone at any time and still stay with the same carrier and contract.

I purchased this phone when I signed up with my carrier but it doesn’t meet my needs.
With an unlocked device you can stay with the same carrier and have a phone that meets your needs.

I have damaged my phone but am not eligible for an upgrade
You can purchase an unlocked device and stay within your contract.

I travel internationally but my phone doesn’t.
All unlocked devices are tri-band and with an international plan will work during your travels.

If I purchase an unlocked phone, are there accessories I need?
If you purchase an unlocked phone, you will still need to own a SIM card and have access to a carrier network. You can usually obtain a SIM card for free from a carrier retail location.

I only copy and paste it from amazon guide page here http://www.amazon.com/gp/feature.html/ref=amb_link_354270402_10?ie=UTF8&docId=1000119471&pf_rd_m=ATVPDKIKX0DER&pf_rd_s=left-1&pf_rd_r=1CB70RPC8W04J5ZF5PNH&pf_rd_t=101&pf_rd_p=1279425422&pf_rd_i=2407749011.