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What's the difference between asking for help sim800C and SIM800A and SIM900A?

The USIM card is an upgraded version of the ordinary SIM card, which can store more address books, short messages and so on. Unused Micro SIM cards are small SIM cards, and ordinary SIM cards can be used by cutting them in proportion. Only a few special mobile phones need to use this SIM card. The iphone 4/ 4s Samsung I9300 is also a Nano SIM card, which is smaller than a Micro SIM card. It is said that iphone5 uses this kind of mobile phone card. I haven't heard of mini SIM card. Generally speaking, they are all SIM cards, but different mobile phones have special requirements for using those special SIM cards.