Some (smart)phones are locked to the carrier (operator) they are purchased from. This means that you can only use that (smart)phone only with a SIM card of the carrier you bought your (smart)phone from. There are many fake websites which claim that they can unlock your mobile device if you pay a certain amount of money, however, this is not true. The suggestions shown below will show you the safest ways to unlock your device, based on the place you bought it from.
Note: Please bear in mind that some USA mobile devices, purchased from Verizon and Sprint, use CDMA, and they cannot be used in other countries out of the USA.
If you bought a second-hand (smart)phone, ask the person who sold it to you where (i.e. in which country) they bought it from.
Maybe your device is not locked to an operator, but experiences issues with the SIM card slot. If it is locked to an operator, you should get a notification, which will inform you that your device is locked when you insert a SIM card. However, if you get an "Insert SIM card" notification, it is possible that the SIM card slot is not working, i.e. your SIM card is not recognized at all.
Check out the device doesn't recognize SIM card guide to learn how to fix this issue.
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