If the user's device or region has some iMessage/FaceTime restrictions, you won't be able to send messages to users in those countries.

You can check the official apple document, to check if there are some restrictions for iMessage/FaceTime in your region. Or restrictions from telecom operators.

Also, at the moment there are some issues with the successful delivery of messages to phone numbers of Chinese telecom operators or devices activated on China's mainland 🇨🇳. We are currently investigating this issue and how it can be resolved. These restrictions are not related to Hong Kong 🇭🇰 or Taiwan 🇹🇼.
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