Kiểm tra xem điện thoại đã được mở khóa chưa Mua thẻ SIM mới. Tắt nguồn điện thoại. Tháo vỏ điện thoại.

Given the above summary, write a detailed text in English for it.
. Many phones are "carrier locked", meaning that they cannot be used with any other carrier's SIM cards.  You can unlock your iPhone or Android if you meet the proper criteria, which depend on your carrier. If your phone is already unlocked, you should be able to use other carriers' SIM cards in your phone. You can usually find SIM cards for specific carriers (e.g., AT&T) in the carriers' respective stores, as well as online and in most tech shops.  Your phone most likely takes a specific size of SIM card, so check the SIM card's supported phones before buying it. If you aren't sure of the SIM card that you should use, you can usually take your phone into a carrier store and have them evaluate it for you. They may even install the SIM card themselves! It's important for your phone to be turned off before you access the SIM card:   iPhone - Press and hold the Lock button on the side of the phone, then swipe the slide to power off switch to the right when prompted.  Android - Press and hold the Power button on the side of the phone, then tap Power off when prompted. If you have an external case on your phone, take it off before you attempt to find the SIM tray, as the SIM tray is integrated directly into the phone's housing.