Samsung SGH-E720

A lot of different things factor into what carrier will have a given phone or not. Most carriers don't want to take just one make of phone because each make of phone has it own perks and down sides but that's not the real kicker.

The first thing that matters is what technology a phone uses. Some of the more common forms are AMPS, CDMA, GMS, and TDMA

AMPS = Advanced Mobile Phone Service.

"The analog wireless transmission standard (technology) deployed in the 1980s in the United States. AMPS operates only in the Cellular frequency band (800 MHz).

AMPS networks are still operating, and offer excellent coverage of most of the country. Many current phones feature AMPS for analog roaming where digital service is not available.

AMPS uses more battery power and emits more radiation than digital modes. Digital modes also have generally better sound quality than AMPS, and provide more features."

CDMA = Code-Division Multiple Access.

"CDMA is a digital wireless technology. It is a general type of technology, implemented in many specific technologies. But the term "CDMA" is also commonly used to refer to one specific implementation: IS-95 - a mobile-phone technology that competes with technologies such as GSM.

CDMA systems have been in commercial operation since 1995. CDMA networks operate in the 800 and 1900 MHz frequency bands with primary markets in the Americas and Asia. IS-95 CDMA systems are sometimes referred to as cdmaOne. The next evolutionary step for CDMA to 3G services is cdma2000."

GSM = Global System for Mobile Communication.

"AT&T Wireless and Cingular, who both used TDMA technology, recently switched to GSM technology. This will narrow the major technologies in the US to two - GSM and CDMA.

The most visible feature of GSM are SIM cards. SIM cards are removable, thumbnail-sized smart cards which identify the user on the network, and can also store information such as phone book entries. SIM cards allows users to switch phones by simply moving their SIM card from one phone to the other."

TDMA = Time-Division Multiple Access.

"TDMA is a digital mobile phone technology. In TDMA, the frequency band is split into a number of channels, which are stacked into short time units, so that several calls can share a single channel without interfering with one another.

Put another way, TDMA allows several devices to share the same frequency band at the same time, by letting each device "take turns" sending digital data. Each "turn" is called a "time slot".

TDMA, as a basic method of transmitting digital signals over radio waves, is the basis for several major wireless standards, including GSM, iDEN, and IS-136. IS-136 is commonly referred to as simply "TDMA"."

Since a given phone manufacturer can make phones with different technologys the tech used to make the phone will play a large part in determining what carrier will use the phone... and then of cource a given carrier will choose what phones it wants to carry based on other things, such as price and even appearence.

An example might be... the Samsung E315... This phone uses GSM technology and opperates on the 900, 1900 bands. Because it is GSM only the ATT/Cingular combine and T-Mobile can use it (as far as the big providers go, some local providers may use GSM, I don't know for sure) but since Cingular and Co. use the 1900, 850 bands, it can be used on Cingular, so it has to be T-Mobile... That's what most of the debate is about... of cource it is all moot if the provider doesn't choose to bring the phone in... again with this example, T-mobile tends to pick Samsung's that have the soft silver coloring, or just light amounts of color, rather than the ones with the shiney black finish or something like that.

So there's an idea what all the fuss is about. You can find more about the different technologies in the glossary (which is where I got those quotes)