When you register a phone number on WhatsApp Messenger or the WhatsApp Business app, you’ll receive a 6-digit SMS code to verify that phone number. When enabled, iCloud Keychain stores your credentials, allowing you to seamlessly register your number on a new device without having to go through the verification process.
Note: Registering a new phone number will require SMS verification even with iCloud Keychain enabled.
All information stored in your iCloud Keychain is end-to-end encrypted. Neither Apple nor WhatsApp can access that information.
Contact Apple support to learn more about iCloud Keychain and how to enable it.
To learn more about verifying your phone number on iPhone, read this article.