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.
Any incoming messages after deleting the WhatsApp account on your Android or iPhone will be queued by the WhatsApp servers and delivered upon successful registration.
Open the WhatsApp (or WhatsApp Business) application on your Android or iPhone.
Navigate to the Settings page.
Select Account.Account Settings
Select Delete my account - Delete My Account
Follow the steps to delete the WhatsApp account for that phone number.
Once the above process is completed, please reach out to firstname.lastname@example.org with your number if you have already registered your business for WhatsApp business access. If not, please complete this step first.