Due to the proximity between Hong Kong and Shenzhen and that mobile signals from the Mainland may spill over to Hong Kong, mobile devices in boundary areas or on some outlying islands in Hong Kong while using local mobile services may sometimes switch to the Mainland mobile network. When this happens, calls made or received and mobile data used for accessing the Internet will be treated as roaming calls and data roaming respectively and charged according to roaming rates.
To avoid your mobile devices from connecting to the Mainland mobile network while you are in Hong Kong, you are advised to set your mobile devices manually to only connect to the designated local network(s). The steps for setting manual network selection may vary with different models of mobile devices. In general, you may follow the steps below:
Alternatively, you may also contact your mobile operator to check whether mobile roaming services could be deactivated / suspended to prevent unintended use of the services.