Cause:

When you save a contact to your phone that is using a Chinese carrier network, the country code for China (0086/+86) will also be saved together with the contact's number. If you are outside China and using a SIM card issued by a Chinese carrier to call a Chinese number, 0086/+86 will be automatically added before the number. If a non-Chinese number with a format similar to a Chinese number is called, the number may mistakenly be identified as a Chinese number. As a result, 0086/+86 will also be automatically added.

Answer:

• If your phone has been installed with a SIM card issued by a Chinese carrier and a SIM card issued by a non-Chinese carrier, disable the Chinese SIM card and then use the other SIM card to make calls.
• When you are saving a contact, save it to the appropriate SIM card according to the service location of the contact's number (This method applies only to versions earlier than EMUI 9.1).
• When dialling a contact's number, touch and hold the number, select Edit before call from the displayed options, and manually change the country code that is automatically added to the number.