What does Unlocking means?
In most countries, cell phones are shipped with country and/or network provider locks. A SIM lock, simlock, network lock or subsidy lock is a capability built-in to GSM phones by mobile phone manufacturers. Network restrict the use of these phones to specific countries and network providers. For example, a T-Mobile phone will only work with T-Mobile service and is locked to all other networks. Unlocking removes the SIM lock or Network lock placed by your wireless carrier. Once unlocked, it can be used with any other service provider in the world such as Rogers or Vodafone.

What are the advantages of unlocking a cell phone? 
Unlocking removes SIM and Network code placed by your wireless carrier. There are many advantages of unlocking a cell phone. The main reason to unlock a phone is to be able to use it with a different SIM card.

When traveling abroad, it can save you a substantial amount of money by simply purchasing a local prepaid sim card and using it with your current unlocked phone instead of buying a new one. It is usually cheaper to temporarily use a foreign network and save ridiculously overpriced roaming charges. Also, an unlocked cell phone have a greater re-sale value compare to a locked one. It attracts potential buyers since it can be used with any network in the world.

Will I have to unlock my phone every time I change SIM card?
No. Once your phone is unlocked, it remains permanently unlocked, no matter what you do to it. Even if you upgrade the software (flash) or hard reset your phone, unless you specially reprogram it back to lock again.

Is cell phone unlocking legal?
It is your phone, isn't it?  Then surely it is as legal that you unlock your phone as it is that you give it away, lose it, break it, leave it turned off, or do just about anything else with it! On November 26, 2006, the U.S. Copyright Office ruled that consumers can legally unlock their cell phones in order to use them with competing carriers.

What is IMEI? 
The International Mobile Equipment Identity or IMEI is a 15 digit serial number unique to every GSM cell phone. It is usually found printed on the phone underneath the battery. The IMEI number can also be retrieved by entering *#06# into your phone keypad.

What is a SIM card?
A SIM card or Subscriber Identity Module is a portable memory chip used in GSM phones. The SIM card allows users to change phones by simply removing the SIM card from one cell phone and inserting it into another. The SIM holds personal identity information, cell phone number, phone book, text messages and other data.

Which companies provide GSM in the USA?
There are two main companies that offer GSM service in the USA.  AT&T (previously Cingular) and the second one is T-Mobile. TracFone also provides GSM service.

All other service providers eg, Sprint, Nextel, Verizon, Alltel, Boost Mobile, Cricket or Metro PCS uses CDMA service and are not compatible with AT&T and T-Mobile network.

Frequency Bands by Country