Mainland mobile phone card, after arriving in Hong Kong, can he receive the information we sent him? Do I need to add any prefixes?

1) If roaming in Hong Kong, Macao and Taiwan is not enabled, the mobile phone will be completely unusable after arriving in Hong Kong.

2) If he has started roaming in Hong Kong, Macao and Taiwan, he can call/send a message to him in China as usual, without adding anything, because his mobile phone number is domestic and only roams internationally, but if he calls you in Hong Kong, you have to add +86.