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When you buy a phone from T-Mobile, Sprint, and AT&T; they offer you a subsidized phone deal in exchange for a 2-year (or 30-months) contract. They also lock your phone, so that you cannot take the phone to another carrier. To use the phone you bought from Verizon on the AT&T network (or vice versa), you'll have to unlock your phone. The US carriers used to lock your phone for years, but with the "Unlocking Consumer Choice and Wireless Competition Act", you have to right to unlock your phone if you meet their "Unlocking Policy".

The easiest way to unlock your phone is by contacting your phone carrier. Each carrier has a different unlock policy, but you have to meet the following requirements to be eligible:

  • Your phone must be paid off.
  • Your account with your carrier must be in good standing.
  • Your phone has not been reported lost or stolen.
  • Your phone must be active on the network for X number of days.
  • Carriers cannot lock a prepaid phone plan for more than 12 months.

To unlock your phone, your carrier will verify your identity by asking your account number, phone number, last 4 digits of SSN, and IMEI, MEID, or ESN number of your phone. The following is the carrier-specific unlock policy.

  • AT&T requires you to be with them for 60 days before they unlock your phone. You can make an unlock request online by visiting Unlock your device page. It generally takes about 2 days to fulfill your request. Prepaid phone plans will require 6-months of service before you can unlock the phone.
  • Verizon does not lock your 4G LTE devices, so they should already be unlocked. Some old 3G iPhones may be locked, but you can either use the unlock code "000000" or "123456", or call their customer service for help.
  • T-Mobile requires you to have service with them for 40 days. For prepaid phones, you can unlock after 12 months or after refilling $100 in your plan. You can request a device unlock by using their device unlock app.
  • Sprint will automatically unlock your phone once it's paid off. Otherwise, you'll require 50 days of service before you can request to be unlocked.

Active duty military people can call the carrier's customer service, and have their phone unlocked without restriction. However, you'll have to prove your active deployment in the US military.

Is it legal to pay 3rd-party company to unlock your phone?

Every phone has an unlock code that removes restrictions on where it can be serviced, and they are available from the manufacturers. Carriers keep unlock codes in their database and unlock phones when they become eligible to be unlocked. Some of those unlock codes are stolen and sold on the black market (or dark web) and monetized by unauthorized persons (or companies). It is illegal for them to sell "stolen" unlock codes. Although it is not illegal for you to buy the code, it may break your contract term with your carrier. It is always best to use your carrier to unlock your phone, and never buy an unlock code from a third party.

How to find out if my phone is locked?

You can check if your phone is unlocked by looking at your phone settings.

  • On iPhone, go to Settings -> General -> About. There is a "Carrier Lock" value showing "No SIM restrictions" if your phone is unlocked.
  • On Android, go to Settings -> Connections -> Mobile Networks -> Mobile Operators. If you uncheck "Select automatically", and see other operators like AT&T, T-Mobile, Sprint, and Verizon; your phone is unlocked.

Unlocked Status
Android ll (Left), iOS 14.4.1 (Right)

What are GSM, CDMA and LTE?

There are two mobile technologies used in 2G and 3G networks, and they are GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access). The newer technology uses LTE, so GSM and CDMA don't matter but for backward compatibility reasons they are still around. In the USA, AT&T and T-Mobile are using GSM while Sprint and Verizon are using CDMA. All US Carriers (AT&T, T-Mobile, Sprint, and Verizon) switched to 4G LTE, but some voice calls were still going through 2G and 3G networks. With US carriers supporting voice over 4G, the distinction will go away and every service will be delivered on 4G and 5G technologies.

One notable difference between the GSM and CDMA is the use of SIM cards. GSM uses the SIM (Subscriber Identity Module) card to identify the subscriber (i.e. phone number) while CDMA doesn't use a SIM card because the information is integrated into the phone. A CDMA carrier like Verizon does not require a SIM card, but they use it to enable LTE so phones that are operating in CDMA technology may have a SIM card slot.

How can I tell if my phone will work on Carrier A?

In order for a phone to work with a particular carrier, the following 3 things must be compatible.

  • Radio technology: GSM or CDMA. With 4G and 5G, everyone will be on the same technology but older technologies are still supported.
  • Frequency bands: Each carrier uses particular frequency bands, and the phone must support that band.
  • Radio channels: Carriers operate in different radio channels, and the phone must support the carrier channel to be used in their network.

Most newer phones support both GSM and CDMA as well as the newer LTE, but frequency bands and radio channels may not be compatible on international borders. Your Samsung Galaxy S20 used with Sprint in the USA, may not work with SKT operator in Korea. Thus, Samsung produces 6 different models of Galaxy S20 meeting different frequency bands and channels.

How can I tell if my phone uses GSM or CDMA? There are a couple of ways you can find its mobile technology.

  1. Look at your "About" settings. On iPhone, go to Settings -> General -> About. On Android, go to Settings -> About Phone. If your setting shows an IMEI number then it's GSM, and if it shows a MEID or an ESN number then it's a CDMA. If it has both, then it supports both GSM and CDMA.
  2. Look up the model number of your phone on Google.

How can I tell if my phone will work with a particular carrier? The next thing that you'll need to check is the frequency band and the channels being used. If you look up the model of the phone and the carrier's spec sheet, you'll be able to find if your phone is compatible with the carrier. Or, you may check with a compatibility check test with your IMEI or MEID.

  • T-Mobile: Enter your IMEI number (GSM), and it will tell you if your phone is compatible. Sprint is now T-Mobile, so new customers will be offered through the T-Mobile GSM plans. The existing Sprint customers can stay on the CDMA network.
  • AT&T: Enter your device type and IMEI, and AT&T will let you know if it's compatible.
  • Verizon: Verizon supports both IMEI (GSM) and ESN or MEID (CDMA) number.

The compatibility check is one piece of the puzzle, and the other puzzle is the "unlocked" status of your phone. Your phone must be unlocked in order for it to be used in a network other than the carrier who locked it.

Does your phone support Dual SIM cards?

There are a few phone models that support dual SIM cards. Dual SIM cards allow you to have two separate phone lines with two phone numbers. You may use one for work and the other for personal, and not have to carry 2 phones. You may also take your phone to other countries, and use the second SIM slot for local carrier service. You may choose to use one SIM as the primary, and the other as secondary so that if you make a call it will use your primary phone line. You may also selectively pick a secondary phone line to make a phone call. You'll receive a call and SMS from both phone lines.

  • iPhone XS, 11, and 12 models support dual SIMs. The primary one is integrated into your phone (eSIM or digital SIM), and the secondary one is populated in a nano-SIM slot. With iOS 13 and later, both phone numbers can send and receive voice, facetime, SMS, MMS, and iMessage. On the iPhone 12 using Dual SIM mode, 5G is not currently supported and will fall back to 4G LTE.
  • Some models of Samsung Galaxy S(20, 20+, Ultra), Z Flip, Note (10, 10+), S(10, 10+) and Galaxy Fold have dual SIM slots depending on the country and carrier.

Samsung Dual SIM
Samsung Dual SIM Slots

There are other vendors like LG, HTC, and Huawei that produce phones that support Dual SIMs. You may have to check their specification sheet to know if a specific model supports dual SIMs.


To determine if your phone is compatible with a particular carrier, (1) the phone must be unlocked, (2) it must support technology whether it's GSM or CDMA, (3) support the frequency band, and (4) support the radio channels. You can contact your carrier to unlock your phone if you are eligible, and check phone specifications to determine if it meets frequency and channel requirements. If you also utilize the dual-SIM, you can have 2 phone numbers in a single phone.

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