T-Mobile cell
phones are indeed compatible with AT&T SIM cards. However, in order to use an AT&T SIM card with a T-Mobile cell phone you have to unlock your new T-Mobile cell phone first. By default, T-Mobile supported cell phones are "locked." This means they are programmed to only recognize SIM cards from the supported carrier, which in this case is T-Mobile. Unlocking the T-Mobile cell phone allows you to use the phone with a SIM card provided by another carrier, such as AT&T.
Instructions
IMEI Number
1. Turn off the T-Mobile cell phone.
2. Remove the battery cover from the back of the phone. Many times this is accomplished by pressing down on the battery cover as you also push forward on the cover to release it from its locks. However, the exact process will depend upon the brand and model of the T-Mobile cell phone you are using.
3. Take out the phone's battery.
4. Make a note of the phone's IMEI number. The IMEI number should be printed on a label taped inside the phone's battery compartment. You need to know this number to obtain the unlock code for your T-Mobile phone.
Unlock Code
5. Contact T-Mobile customer care department at 1-800-937-8997 to obtain your T-Mobile phone's SIM unlock code. T-Mobile provides SIM unlock codes to existing and former customers for free, provided certain conditions are met. For current T-Mobile post paid plan customers, you must have at least 40 days of active service with T-Mobile in order to obtain the SIM unlock code for your T-Mobile phone from T-Mobile customer care. For current T-Mobile pre-paid plan customers, you must have at least 60 days of active service with T-Mobile and a current account balance of at least $10 in order to obtain the SIM unlock code for your T-Mobile phone from T-Mobile customer care. For former T-Mobile customers, T-Mobile customer care will supply you with the SIM unlock code for your T-Mobile phone provided they have it on record or can obtain it from the phone's manufacturer.
6. Use a third-party cell phone unlock code finder service if you are unable to obtain the SIM unlock code for your new T-Mobile phone from T-Mobile's customer care department. Many of these third party companies charge a relatively small fee for their services. However, there are also a few services available online which are free of charge (see the links in the Resources).
7. Follow the cell phone unlock instructions included with the unlock code you received. At some point in the instructions you will be directed to insert the foreign SIM card into your cell phone. When instructed, insert the AT&T SIM card.
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