While both carriers run GSM networks and are both in the process of updating them to support 21Mbit/s High-Speed Packet Access (HSPA)+, they operate in different, incompatible frequency bands. AT&T runs on 850MHz and 1900MHz frequency bands for uplink and downlink, while T-Mobile operates on paired 1700MHz/2100MHz AWS spectrum.
As such, the only smartphones that roam between AT&T and T-Mobile's 3G networks are world phones, designed for international travelers.
All of AT&T's most popular models, like the Apple Inc. (Nasdaq: AAPL) iPhone 4 and iPad 2, lack support for 1700MHz, meaning they will only bust back to T-Mobile's compatible EDGE network when the two become one.
T-Mobile customers, on the other hand, will have 27 devices to choose from, including the carrier's latest smartphone, the myTouch 4G Slide. Other HSPA (7.2Mbit/s) or zippier HSPA+ devices at their disposal include:
- High Tech Computer Corp. (HTC) (Taiwan: 2498) Sensation 4G and HD7
- LG Electronics Inc. (London: LGLD; Korea: 6657.KS) Optimus T
- BlackBerry Curve 3G and Bold 9780
- Dell Technologies (Nasdaq: DELL) Streak 7
- Motorola Mobility LLC Defy and Cliq 2
- T-Mobile myTouch 4G, myTouch 3G Slide and MyTouch 4G Slide
- Nokia Corp. (NYSE: NOK) Astound
- Samsung Corp. Dart, Gravity T, Smiley, Gravity 3, Gravity Smart, t259, Vibrant and Galaxy S 4G
- T-Mobile Rocket 2.0, Jet 2.0, 4G Mobile Hotspot, Rocket 3.0, Sidekick 4G and G2x
Purchasing world phones with the dual-frequency support makes more sense for T-Mobile customers anyway, since they are the ones getting kicked off their frequencies once the merger closes. But, the big holdout remains, of course: When will Apple build an iPhone that supports T-Mobile's frequencies as well? (See Could AT&T/T-Mobile Deal Mean an HSPA+ iPhone?)
Light Reading Mobile will have more on that soon.
— Sarah Reedy, Senior Reporter, Light Reading Mobile
Because of component space, energy consumption, and deveopment and component cost. If you want a bigger phone that costs more and has less batter life then you can certainly add these components. Alternatively you can make more different phones, one for each frequency/technology, which then adds development costs and manufacturing cost because you have to keep several production lines running and the inventory is not fungible anymore. Right now Apple has to predict how many iPhones 4 16MB it sells, not how many iPhone 4 16MB 850/1900 MHz HSPA, iPhone 4G 16 MB 900/1900 MHz HSPA, iPhone 4G 1850 HSPA+, iPhone 4G 700 MHz LTE, iPhone 4G 2100 MHz LTE, etc it will sell. Much easier to predict one number than 5 or 10...