An unlocked phone is the key to getting service from an alternative carrier.

We talk a lot about using MVNO carriers as a way to find a cheaper alternative for service. It's something that's gaining popularity and while it isn't exactly mainstream just yet, it's getting there. We think that's great because having even more choices can't be a bad thing. But in most cases, before you can try any other service provider, you'll need to have a SIM-unlocked phone. That means you can use a SIM card from any company and as long as the hardware is compatible with the network, it just works.

While some of us already know if our phones are unlocked or how to check, here's what you need to know if you are looking for a little help.

More: Best Unlocked Phone

Where did you buy it, and how?

In the U.S., Verizon is the only carrier who is consistent with selling all their smartphones unlocked. What we mean here is that if you walk into a Verizon store or use their website and buy a phone made for Verizon with their name on it, it's probably going to be SIM unlocked. Note that this doesn't mean you should buy a Verizon branded phone to use on a different network because SIM unlocked doesn't mean fully compatible. Just that Verizon is OK with you putting another SIM in the phone and letting the hardware try to connect to a different network.

The other three networks, as well as Best Buy or other third-party carrier resellers, are not as reliable when it comes to selling unlocked phones. Some are, some aren't — but every one of them can be unlocked, because that's the law.

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If you bought your phone from Amazon or B&H or another online retailer, it told you in the listing if it was unlocked. If you don't remember, a quick call or email to customer service will help you find a copy of the listing to double check. Of course, if you bought your phone directly from the manufacturer, if it was listed as unlocked it will be and there is no need to check. All phones sold directly from Google and Apple (if paid in full at time of purchase) are unlocked, and many phones from other manufacturers are sold directly and SIM unlocked, too.

For the rest of the phones out there, there's really only one fool-proof way to check — try it.

Use a SIM card from a different carrier

You'll see references to the website imei.info that claim you can check online. While that might be true, the service costs money and has been incorrect five out of five times I have tried. I'll come right out and say it — don't bother.

You can call your carrier or the carrier or MVNO you're thinking of switching to. There is a database of IMEI numbers and it says which phones are unlocked or blacklisted. You might not have any luck here, though, because the person answering the phone might not be able to give you that information. Either because they don't have access to it or they aren't allowed to tell you over the phone. It's worth a try if you don't have access to a SIM card from a different company, though.

If you do have access to a SIM card from a different carrier, checking is pretty simple.

  • Make a phone call using the right SIM card if possible. This is to check that the phone is working properly. Call your mom and tell her hello; she'll appreciate it.
  • Shut off the phone and remove the SIM card. Look in the manual or online if you're not sure how to do this, and if you need a SIM card removal tool, a paperclip works fine.
  • Put the different SIM card in the phone and power it back on.
  • If you're greeted with a dialog box telling you to enter an unlock code, your phone is SIM locked to the carrier it was originally purchased from.
  • If you don't see this, check that it says you have service. If you do, your phone is probably unlocked. Try making another phone call. Mom won't mind two calls on the same day.

If you can't call using the different SIM card, you'll probably get a recorded message with some error code. you can jot down the code and check online to see why the call can't be completed so you know if it's because of a SIM lock or because of incompatible hardware.

If your phone is SIM-locked, don't fret. As mentioned, a carrier has to unlock a phone unless you owe money on it according to U.S. law. Give them a call and they will help you.

When you don't need a SIM-unlocked phone

At the top of this article, we said in most cases you do need a SIM-unlocked phone to try another carrier. That's true, but there is one popular use-case where it doesn't matter: using a T-Mobile branded phone on an MVNO using T-Mobile's network.

T-Mobile actively assists resellers, and they don't require a reseller (that's what an MVNO is) to require you to have an unlocked phone to connect. If your phone says T-Mobile on it or on the box, it will work on most MVNOs that use T-Mobile's network. [And there are a lot of them](Complete List of T-Mobile MVNOs).