Why do Hong Kong mobile phones have two zeros before sending SMS numbers to the Mainland? What format should the original number be?

I remember, there are more than two zeros:

00 1-86- 139 12345678

Where 00 1 is the prefix of international calls or short messages sent by Hongkong, and 86 is the country code of Chinese mainland.

From Chinese mainland to Hongkong:

00-852-98765432

Among them, 00 is the prefix for Chinese mainland to make international calls or send short messages, and 852 is the country code of Hongkong.

The difference is that the prefix of international calls from different countries is different, which depends on the local telecommunications standards.