The WhatsApp feature “Click to Chat” exposes mobile phone numbers on the web and Google’s search engine is able to index them. You can try it using the search string
The phone numbers are simply as part of an URL string (https://wa.me/<phone_number>) and available in plaintext. Facebook claims that this not a bug:
Our Click to Chat feature, which lets users create a URL with their phone number so that anyone can easily message them, is used widely by small and microbusinesses around the world to connect with their customers.
However, not all users were aware of this feature. While some have no problem with that (“my phone number is public anyway”) others were surprised that their number has been indexed by Google.
This is similar to a different WhatsApp privacy problem from earlier this year, when WhatsApp group were indexed by Google’s Search Engine.
Basically, if you are a business, you must be very careful when using WhatsApp. It’ certainly better to consider alternatives like qnews or different messaging tools.