Throughout the world, different frequencies are used to send and receive wireless calls within each country. The phone manufacturers (Nokia, Motorola, Samsung, etc?) then produce phones that will work within each countries network(s). To complicate the matter in the United States; AT&T, Verizon, T-Mobile, Sprint, and the other carriers, use one of four different technologies to transmit a call to your phone. The different technologies are known as GSM, CDMA, TDMA, and I-Den. GSM and CDMA are supported by most carriers, while I-Den and TDMA are being phased out. You can think of these technologies as operating systems for your phone.

For the average user, it's not very important what technology is being used by which carrier.  Contrary to what you hear on commercials, every carrier uses and shares the same frequencies and the differences between technologies has little effect on the quality of the calls you make and receive.  The more important factor is which carrier has better service in the particular areas in which you plan to use your phone ? this is usually more dependent on which carrier has more cellular towers in the areas than anything else.  Information regarding cellular protocols and frequencies is more important if you already have service with a carrier and you decide to purchase a phone by itself from an independent source.

All phones being used in the U.S. should accommodate the 800/850 and 1900 bands to operate in most areas well, except on the T-Mobile and SunCom networks which operate solely on the 1900 bands.  The 800 and 850 bands are actually the same, so don't be confused by the two different numbers.  850 just means that the phone is GSM-only compatible, while 800 can be a GSM, CDMA, I-Den, or TDMA compatible.  Any phone which lists the 800 or 850 + 1900 band in the phone specifications will work well in the U.S., as long as it is also supported by the carriers GSM, CDMA, I-den, and TDMA networks.  Note that phones that are 1900 compatible but lack the 800/850 band will operate in some areas of the U.S. but not others; major metropolitan areas in particular tend to offer better 1900-band coverage.

Carrier Technology and Frequency/Band Chart

Adding a replacement phone to your current carrier, U.S. only.

• The phone must be compatible with the 800/850 and/or 1900 bands
  
  
• The phone's protocol technology, meaning CDMA, GSM, I-Den, or TDMA must match the technology your carrier is using.
  
  
• Make sure the phone is unlocked.  This allows you to add the phone to your carrier's network.  Some websites also provide free or for-fee information on unlocking phones.
  

Frequency Bands Used throughout the World

U.S Bands ? 800/850, 1900 (CDMA, GSM, TDMA, or I-Den), 2100 CDMA 2000/EVDO
Europe ? 900, 1800, 1900 (GSM Standard) and 2100 WCDMA/UMTS
Japan ? 900 (JTACS - Japan Total Access Communication Service)

What is GSM 850

GSM 850 is simply GSM technology operating in the Cellular (800 MHz  850 MHz) frequency band. Both the technology and frequency band have been around for a long time, but only in 2002 were they combined.

In the U.S. prior to 2002, GSM technology only operated in the PCS (1900 MHz) frequency band.  GSM 850 addresses the needs of carriers with Cellular licenses switching from other technologies to GSM.

Before the existence of GSM 850, the Cellular band was commonly referred to as the 800 MHz band. 850 MHz implies a different frequency band, but this is not the case. 800 MHz and 850 MHz refer to the exact same frequency band. 850 is technically a more accurate description of the frequency range, although 800 is still common when referring to CDMA and AMPS (analog) technology.