Yes, it is possible to use a number that is already in use on WhatsApp/WhatsApp Business for WhatsApp Cloud API. Before proceeding, there are some important points you should consider:
1. You will need to delete the currently used WhatsApp account before starting the procedure. Once the account is deleted, it will not be possible to recover the chat history or information present in the deleted account.
2. Once the transition from a traditional WhatsApp account to a WhatsApp Cloud API line is completed, it will not be possible to use the account in question with the native WhatsApp interface (on the mobile app or on the web). Management of this WhatsApp account will be exclusively done through the Callbell interface (on PC or on the Callbell mobile app).