A burner phone is a simple, often inexpensive cell phone that you do not link to personal data or accounts and that you can leave behind or dispose of in case of doubt without losing anything.
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Especially when entering repressive countries such as the USA, Russia or China, you must expect your device to be searched. A burner phone can help you protect sensitive information because you don't have it with you in the first place.
If you do not allow your device to be searched, you may face refusal or detention.