Can I have my phone and use it on AT&T too?

Unlocked phones don't belong to any specific carrier, but if they don't support the same radio bands as the carrier of your choice, then you're out of luck. Different carriers will use different radio frequencies or bands to transmit their mobile signal. These bands are all numbered; for example, the band AT&T primarily uses is 17.

Why can't I just buy any phone and have it work on any carrier?

If your phone does not support a particular band, especially a carrier's main LTE band, then you may not be able to access that carrier's LTE network. For instance, the OnePlus X is an affordable unlocked phone, but it does not support band 17, so you won't get LTE on that phone in some places. It is important to make sure that your unlocked phone can support a carrier's main LTE band before signing up for servie. What good is having a smartphone if it's only running on the smartest network in the dumb network class?

What the heck's a radio band?

A radio band is a radio frequency used by a mobile network to transmit data. Most carriers use a few bands at a time, so that if one frequency can't reach you, hopefully another one can.

Certain carriers only use certain bands because – if you think of it like a highway – if all carriers use the same bands, then you'd have a MAJOR traffic jam on your hands and cellular data would slow to a crawl.

Thanks, professor, but what bands does AT&T use?

AT&T presently supports 10 different bands, so it's best to check the specs of the phone you're considering before buying.

The bands AT&T uses are separated into three categories: 2G, 3G, and 4G(LTE). The carrier plans to decommission its 2G bands by January, 2017.

2G bands

  • 5
  • 2

3G bands

  • 2
  • 5

4G/LTE bands

  • 2
  • 4
  • 5
  • 17 — AT&T's main band
  • 29
  • 30

You'll want to make sure your unlocked phone supports band 17 before you sign up with AT&T. Otherwise, or you'll feel like you're living in the stone age phone age.

Things you might be missing out on

If you have an unlocked phone, that means it wasn't made for AT&T. If that's the case, then you may be missing out on some key features that AT&T provides, like Wi-Fi Calling or Voice over LTE (VoLTE). You may want to take this into consideration if you already rely on these features.