AT&T MNC Priority List
GSM carriers use the Mobile Network Code (MNC) to prioritize which carrier they will access when the home carrier is not available. AT&T phones, when given a choice, will access these networks in the following order. Updated Feb. 22, 2008

• MCC-MNC | Carrier
  
  310-410 AT&T
  310-380 AT&T
  310-900 Texas Cellular
  310-450 Viaero Wireless
  310-300 T-Mobile
  310-560 AT&T
  310-830 Caprock Cellular (TX)
  310-800 T-Mobile
  311-080 Pine Cellular
  310-100 Plateau Wireless
  311-901
  311-030 Centennial Communications
  310-890 Unicel (RCC)
  310-770 Iowa Wireless
  310-950 XIT Wireless (TX)
  310-340 Westlink Wireless
  310-150 AT&T
  310-590 Alltel
  311-040 Commnet Wireless
  310-320 4 Corners Wireless (Cellular One of NE Arizona)
  310-610 Epic Touch (OK)
  334-020 Telcel (MX)
  311-140 Sprocket Wireless (MBO Wireless, OK)