Why is the same number preceded by +86 when calling or texting, but sometimes it is not? (If it is the number in my address book, there is no such phenomenon at all. )

If the other party's number is stored in the mobile phone, that name will be displayed when making a phone call or sending a text message. If you don't save, +86 will be displayed. Has nothing to do with the attribution of the other party's number

Because China's mobile network, Unicom. Telecommunication networks are in line with international standards. 86 is the international area code for Chinese mainland. If you call China's mobile phone from the United States, you should add 0086. .....................................................