Many smartphones purchased from U.S. wireless carriers come locked onto that carrier’s network. If you’re trying to move to a new carrier, and you want to use your current phone rather than buy a new one, you’ll have to unlock that phone before activating it on a competitor’s network.

If you’re with one of the four largest U.S. carriers — Verizon, AT&T, Sprint or T-Mobile — unlocking your phone to enable it to work on any network should be relatively simple.

Last year, all four companies agreed to unlock a carrier-subsidized phone if the customer has fulfilled the requirements of the accompanying contract or installment plan. The companies are required to notify users when their devices are eligible for unlocking.

So if you bought your phone from the carrier at full price without any subsidy, you can unlock it one year after the phone’s initial activation on the network. If you bought a subsidized phone on a two-year contract, and that contract is up for renewal, you can unlock the phone. If you paid for your phone on an installment plan and completed paying for that plan, you can unlock the device.

In all cases, your account must be in good standing, which means there should not be any outstanding bills or illegal usage. The four companies agreed to unlock phones of former customers as well.

If you meet these criteria, you can call your carrier to get your phone unlocked. We’ve listed the numbers for the major carriers below.

Keep in mind that even if your phone is unlocked, there’s no guarantee it will work on another carrier’s network. Check with the other company first to make sure that your phone is compatible with its network.

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Is my phone already unlocked?

People sometimes search online to find a way to check if their phone already is unlocked. Some websites may charge a fee or ask for personal information to tell you whether your phone is unlocked. Don’t fall for it.

There’s one free, surefire way to know if your phone is unlocked — it’s a minor hassle and should take a few minutes.

Borrow a phone from a friend whose wireless carrier is different from yours. Pop out your SIM card and then put your friend’s SIM card into your phone. If your phone gets service, that means it’s already unlocked. If not, move onto the next section.

How do I unlock my phone?

Each carrier requires a phone call or a visit to its webpage to start the unlocking process. When you call, ask the customer service person to “network unlock” the phone for you.

Here are some numbers and websites to get you started. Read the full policies for details:

Verizon: Call 800-711-8300. Newer 4G LTE Verizon phones come with an already unlocked SIM card slot. Older Verizon phones that operate only on Verizon’s CDMA network may not be compatible with other networks, even if they’re unlocked. (Verizon’s unlocking policy.)

AT&T: You can start the process of unlocking your device on this webpage. (AT&T’s unlocking policy.)

Sprint: Call 888-211-4727 or start a webchat. Sprint’s network operates on CDMA technology, so even if its phones are unlocked, they may not work on other networks. But you might be able to move the phone to one of Sprint’s MVNOs — or mobile virtual network operators — such as Boost Mobile or Virgin Mobile. (Sprint’s unlocking policy.)

T-Mobile: Call 877-746-0909 or start a webchat. (T-Mobile’s unlocking policy.)

Boost Mobile: Call 888-BOOST-4U (888-266-7848). Boost’s network uses CDMA technology, which is not easily compatible with other networks. Even if your Boost phone is unlocked, you might not be able to take it to another carrier. (Boost’s unlocking policy.)

Cricket Wireless: Call 800-274-2538 or start a webchat. Cricket will unlock your phone six months after it’s activated. (Cricket’s unlocking policy.)

MetroPCS: Call 888-863-8768. MetroPCS will unlock your phone 90 days after its initial activation. (MetroPCS’s unlocking policy.)

U.S. Cellular: 3G phones can be unlocked by calling 888-944-9400; 4G LTE models may already be unlocked or need to be unlocked at a U.S. Cellular store. (U.S. Cellular’s unlocking policy.)

Virgin Mobile: Call 888-322-1122. Virgin operates on a CDMA network like Boost, so Virgin device’s may not be compatible with other networks. (Virgin’s unlocking policy.)

Even if your carrier isn’t listed here, the steps to get your phone unlocked generally are the same: Call the company, go to its website or visit a store. Unlocking your phone is key before moving to another carrier — and you might also get more money for an unlocked phone if you’re trying to sell it.

Stephen Layton is a staff writer at NerdWallet, a personal finance website. Email: slayton@nerdwallet.com.

The article How to Unlock Your Phone originally appeared on NerdWallet.