What's the best phone to have when dumping your carrier?

Carrier contracts are dead, and you have more flexibility in choosing a provider than ever before. But is it really that easy?

Not quite. You may still be stuck with a provider you aren't happy with, because your phone isn't compatible with another carrier's network. Luckily, the device landscape is changing.

In this edition of Ask Maggie, I explain.

Dear Maggie,

With all the carriers eliminating their phone subsidy programs and more or less doing away with contracts, we users are faced with owning a phone that may be compatible with only one specific carrier. To switch to another carrier probably means having to buy another phone even though the one we have is perfectly fine. Are there phones out there that are compatible with all the major carriers' technology?

Thanks,
Bill
former rocket scientist

Dear Bill,

You are absolutely correct that not all devices are compatible on all networks. Even if you're not bound by a contract, you could still be stuck if the new $650 device you bought works only on a specific carrier. But things are changing. There are several devices available that work across all four major carrier networks.

Enlarge Image

Before I get to the devices, let me explain why this is an issue. When the wireless market was born, US wireless operators split into two camps. One camp (now AT&T and T-Mobile) supported the globally accepted technology called GSM. The other (now Verizon and Sprint) chose another, less widely deployed technology called CDMA.

The two incompatible technologies are still used primarily for voice and text messaging service.

When 4G technology rolled around, major carriers throughout the world agreed to use the same technology, called LTE. But because different carriers use different radio frequencies to deliver the service, not all phones with LTE are compatible.

Fortunately, device makers have been working on resolving these issues. Now several devices include both GSM and CDMA technology, which makes voice and text messaging compatible across all four major networks. There are many handsets that also include several frequency bands for LTE, making them compatible for high-speed Internet access across all four major carriers.

To be sure that the device you own will work on another carrier's network, you should check the specifications to see which LTE frequency bands are supported. I've put together this chart which shows the different LTE bands supported by the four major mobile carriers in the US. Sprint also offers this helpful FAQ for people bringing devices to their network.

Which phones will work across all four major US carriers?

Now for the answer you've been waiting for. Here's a list of devices, from three manufacturers, that should work across AT&T, Sprint, T-Mobile and Verizon.

Apple
iPhone 6S, iPhone 6S Plus, iPhone 6, iPhone 6 Plus

Lenovo
Moto X Pure Edition

Google
Nexus 6, Nexus 5, Nexus 5X, Nexus 6P

Notably missing from the list above are the popular Samsung Galaxy S and Galaxy Note devices. Unfortunately, Samsung still slightly customizes its phone for each carrier. Check the specs of your particular device or ask a carrier representative about compatibility issues before you decide to switch.

Locked vs. unlocked

One other thing to keep in mind is that even if your device is technically compatible with another carrier's network, it must be unlocked. Verizon is the only US carrier that sells 4G LTE devices unlocked out of the box. All the other carriers, even if you're paying full price for your device, will have software locks on their phones.

That said, carriers are required by the Federal Communications Commission to unlock devices that customers own in full. The operator may still have other criteria before it provides you with an unlock code. For instance, the devices you're seeking to unlock must not be reported lost or stolen and the account holder should be in good standing. T-Mobile also states in its policy that the device must have been active on its networks for at least 40 days.

Here are links to unlock policies for AT&T, Sprint and T-Mobile.

The Moto X Pure Edition and the Google Nexus devices are sold only as unlocked devices. This means that you can't purchase them through a carrier, and by default they're unlocked and will work across carriers. Apple sells an unlocked version of its iPhones. The versions listed above sold through various carriers should also work across carriers once they're unlocked.

It's important to note that other phone companies sell "unlocked devices" -- think budget vendors like ZTE or Alcatel OneTouch -- but these unlocked phones typically work only on AT&T and T-Mobile.

The bottom line

If you're in the market for a new phone, and you're looking to future-proof your purchase, the list of devices above is a good place to start.

Ask Maggie is an advice column that answers readers' wireless and broadband questions. If you have a question, I'd love to hear from you. Please send me an e-mail at maggie dot reardon at cbs dot com. And please put "Ask Maggie" in the subject header. You can also follow me on Facebook on my Ask Maggie page.