T-Mobile US Inc. customers won't automatically get the iPhone 4 if AT&T Inc. (NYSE: T)'s acquisition goes through, but they will have a lot more 3G handsets to choose from to ride both carriers' networks than AT&T's customers will.

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
  
  
  

Most of the merger talk has focused on how T-Mobile will bolster AT&T's Long Term Evolution (LTE) deployment plans, but the carrier will also have to integrate these legacy phones on AWS spectrum. While AT&T may commission new dual-frequency handsets to support both networks in the future, its existing smartphones won't benefit from the bolstered network capacity.

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