Migrating the phone number of an existing WhatsApp Android, iOS account or a WhatsApp Bussiness App account is possible. Please indicate to your account manager that the number you intend on using is already in use, or send an email to firstname.lastname@example.org after registering for a WhatsApp Business account with us.
When the time comes to activate your WhatsApp Business Account, you will have to unregister your old account yourself, before the technical activation can take place.
Note: Once a number is in use for WhatsApp Business, it cannot be downgraded to WhatsApp for consumers !!!
If your number is already registered with another WhatsApp Business API partner then it cannot be moved using the process detailed here. Please contact us, but understand that we will be highly dependent upon WhatsApp in cases like that.