There are many types mobile phone technologies in the world today. Each type uses different frequency bands. In the United States the most popular bands used are PCS, Cellular and iDEN. The other emerging ones are GPRS, EDGE and UMTS. Mobile Phone Carriers might use one or a combination of the frequencies ranges for their services. Example is Verizon Wireless uses PCS and Cellular. 

The frequencies ranges are:

PCS stands for Personnal Communication Service. Sometimes refered to as the Digital Mode, it represent the signals that transmit between frequencies of 1850 to 1992 MHz (megahertz). Since PCS covers the 1850 to 1992 MHz spectrums, it employs 2 bands mode i.e. the 1800 and 1900MHz. These frequencies are used by Mobile Phone Standards like CDMA, TDMA and GSM. Most carriers including AT&T, Telus, T-Mobile, Qwest, Verizon Wireless, US Cellular, Alltel and Bell Canada uses PCS network in combination with Cellular.

Cellular transmit between 824 to 994 MHz (800MHz and 900MHz band mode). CDMA and TDMA employ these frequency ranges. The analog mode used by Carriers like Verizon Wireless, Sprint and Alltel reside in this frequency range.

iDEN transmit on the 806 to 866 MHz frequency ranges. This range is used by carriers like Nextel/Sprint, Mike, Southern LINC and Boost.It's a stand alone mode.

Mobile phones are built with a specific frequencies band mode. Most CDMA are 900 and 1900MHz. GSM phones could be Dual-Mode, Tri-Mode or Quad-Mode. Dual-Mode is with 800 and 1900MHz bands. Tri-Mode is with 900, 1800 and 1900MHz bands. Quad mode is with 800, 900, 1800 and 1900 MHz bands.

GSM Carriers like T-Mobile and AT&T/Cingular sell their mobile phones locked with their access specifications. These are referred as to as Locked Phones. CDMA Carriers like Verizon Wireless, Sprint and Alltel does the same with their mobile phones but since CDMA is not a widely used standards worldwide, not many consumers want to unlock their CDMA mobile phones. Some do flash the mobile phones software so as to enable them to use it for other CDMA carriers including NVMO like Disney, Liberty, Vedafone, Orange etc.NVMO are independent operators that do not owe their own signal towers. Instead they buy block of signal time from Carriers.

Why do you need Quad-Mode GSM phones? Different part of the world use different frequencies bands thus your Quad phones will work anywhere in the world.

How do you unlock a locked carrier phone? If you are travelling out of country you can call your carrier (T-Mobile or AT&T/Cingular) to provide you the access code to your phone. Be ready to give them your phone IMEI number that is located at the back of your phone behind the battery.

Equipped with this access code and the instruction provided, you could unlock your phone for international use within minutes. Most carriers will require you to be in service with them for a specific time and in good standing with them before giving you the access code. Good standing means you have paid your bills and not passed due. 

Alternatively, if you are in a rush and want to unlock your phone right away without the hassle of contacting your carrier, you can look up one of your local cellular stores that might do it for you for a fee. We also hear rumors that there are some software out there that could do it. Carrier do not wnat to unlock their phones at will to prevent trans-shipping. They will even go as far as to double lock their more expensive phones since they might have to subsidize it to be more attractive for new subscribers.

Your other option is to buy a virgin Unlocked Phones are phones that were manufactured without the carrier access specifications. Thus you can use any carrier SIM card to on these unlock phones. Please check with your carrier before doing so as certain smart SIM cards might kick you out of your network when using an unlocked phone that is not flashed with the carrier software.

Description of terms used in the guide.

CDMA stands for Code Division Multiple Access. It uses several potocols in second generation (2G) and third generation (3G) wireless communications. CDMA is a form of multiplexing which allows numerous sighnal to occupy a single transmission channel optimizing the use of available frequencies. This technology uses both the PCS and Cellular signals.

TDMA stands for Time Division Multiple Access. It's  a technology that divides each cellular channels into three time slots in order to increase the amount of data that can be carried.

GSM stands for Global System for Mobile communication. First lauched in Finland in 1991, it digitilizes and compress data and then sends it down a channel with two other treams of user data each it its own slot. It operates at either the 825~994 MHz or the 1850~1992 MHZ freqeuncies bands. Present on this technology are SIM cards. GSM is the most popular and widely used standards used by most carriers in the world including T-Mobile and AT&T/Cingular. 

SIM Card stands for Subscriber Identity Module. As know as Smart Card. It's a small piece of removable chip that is embedded in your phone that holds network access configuration of your carrier.

Trans-Shipping is an illegal practice whereby.an operator when discovering that a particular carrier locked phone is more expensive in other part of the world decides to purchase, unlocks the phone and then sells it abroad. The reason for the disparity in prices is because some carriers subsidize their phones for it to to be more attractive to new subscribers. This allows profit taking for illegal operators.