Steps to use phone number that has been registered with Whatsapp Business before

To apply for WhatsApp API Verification, you must use a number that does not have an account on any WhatsApp apps.

You can use a phone number that is already registered in the Android, iPhone, or Business application versions of WhatsApp.

However, in order to register this phone number on WhatsApp Business API, you will need to follow the steps below to delete the WhatsApp account associated with that phone number.

Once a phone number is registered on the WhatsApp Business API, it cannot be used for WhatsApp Business or the WhatsApp consumer app. A phone number can be upgraded to the WhatsApp Business API, but not downgraded.

To delete your account

1. Open WhatsApp.
2. Tap More options > Settings > Account > Delete my account.
3. Enter your phone number in full international format and tap DELETE MY ACCOUNT.
4. Select a reason for why you're deleting your account in the dropdown.
5. Tap DELETE MY ACCOUNT.

Deleting your account will:

1.  Delete your account from WhatsApp.
2.  Erase your message history.
3.  Delete you from all of your WhatsApp groups.
4.  Delete your backup.

Information from Official WhatsApp documentations:  https://developers.facebook.com/docs/whatsapp/guides/phone-number/#migrate