You can use a phone number that's already registered in WhatsApp Business Application; however, if you add that phone number to your Business Manager, you'll see a "Phone Number is Invalid error". Don't worry, simply follow the 4 easy steps of this article to delete the WhatsApp account associated with that phone number.
THERE WILL BE DOWNTIME DURING A NUMBER MIGRATION
Keep in mind that any incoming messages during this time will be queued by the WhatsApp servers and delivered once the registration is successful. Previous messages stored in the APP will not be visible in "Inbox".
Go to the 'Settings' page withing the WhatsApp Business Application and click on 'Account'
Select ‘Delete my account’.
Confirm your country code, phone number, and click on 'Delete my account'.
Voilà! Now simply send your WhatsApp phone number to the MessageBird Support team at firstname.lastname@example.org. We'll now be able to register your number directly with WhatsApp.