Hi,
This API is not checking against a DB or something like this, there are no false negatives. We check if the number is registered on whatsapp on each request you make.

If the number is registered, and the API returns the number is not, 100% of the times the reason is a malformed phone number.

If you want to make sure the number is correctly formed, please read this:
https://faq.whatsapp.com/640432094208718/?helpref=uf_share and also this guide (end of the document): https://rapidapi.com/inutil-inutil-default/api/bulk-whatsapp-validator/tutorials/how-to-use-whatsapp-validator%3F

The last advice would be to use this API (there’s a free tier) to make sure the number exits, it’s a mobile number, and get it in the right format: https://rapidapi.com/inutil-inutil-default/api/phone-validator-api