If you buy a cellphone from your carrier, it’s almost always locked to their network. You can’t just stick an AT&T sim card in a T-Mobile Verizon. Or can you? If your smartphone is out of contract you can demand they unlock it and do just that. Let’s dig in.

Premium cellphones are expensive—the cheapest iPhone 8 costs $699—so for a lot of people, it makes sense to sign up to a carrier plan that includes a subsidized cell phone. You pay $200 now and get a brand new iPhone 8 in your hand; over the 24 months of your contract, you pay off the other $500 in addition to whatever your plan costs.

The problem is that if you buy a phone on contract from T-Mobile, it will only work on T-Mobile’s network even after your contract ends; you can’t just grab a SIM card from AT&T and stick it in. That is, at least, until you ask them to unlock it.

All the national level providers in the US—think Verizon, Sprint, AT&T, and so on—are mandated by law to unlock your phone once you’ve fully paid off the device subsidy, your contract ends, or you’ve paid an early termination fee. This means that you’ll be able to use it on other networks and even internationally.

An unlocked smartphone has a couple of benefits over a locked one. First, they sell for more cash. Since it’s free for you to unlock it once your contract is up, that’s some pretty easy money. Second, if you want to pass it on to a friend or relative, they don’t have to be on the same network as you. No one likes receiving a gift that involves filling in forms and paying a contract termination fee. Finally, if you plan on keeping your phone, you can hop contracts to save cash.

The one snag with unlocking your phone is that it might not be able to work on all networks. In the US, there are two competing standards: CDMA and GSM. Sprint and Verizon use CDMA while AT&T, T-Mobile, and almost all international carriers use GSM. If you buy a phone from a CDMA carrier, there’s a very good chance it is also compatible with the GSM network; an iPhone 8 bought from Verizon and then unlocked, for example, can be used on T-Mobile’s network.

The opposite, however, isn’t necessarily true. An iPhone 8 bought from T-Mobile is compatible with all GSM networks around the world but it’s not compatible with any CDMA network. It doesn’t have the necessary internal components. It’s worth checking which standards your phone supports before committing to any new contracts.

If your contract is up, you are legally entitled to get it unlocked for free. You shouldn’t run into too much difficulty, but if a customer service rep tries to get you to sign up for a new contract or the like, tell them to take a hike and unlock your phone as the law requires.

Each of the major carriers has slightly different unlock policies:

While I’ve been focussing on the US in this article, other countries often have similar laws. If your contract is up, contact your provider and ask how you can get your device unlocked.

Once you’ve completed the unlock process the simplest way to check to see if the phone is unlocked is to pop a SIM card from a different carrier in it. If it’s still carrier locked then you won’t get a cellular signal from the alternative carrier’s network and, on most phones, you’ll get an error message indicating the SIM isn’t compatible. If you don’t have a SIM on hand, you can always call a competing but compatible carrier (e.g. you have AT&T so you call T-Mobile) and ask them to check if your IMEI unlock my cell phone number for your phone is clear to activate on their network.

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