Migrating phone numbers
These steps are only for numbers registered as WhatsApp Business API accounts. If you are not sure your client's number is registered as a WhatsApp Business API, try messaging it.
If the Call icon
appears in the top right of the app, it means that this is a WhatsApp Business App account. In this case, you should follow the steps outlined here.
When a client wants to migrate an existing WABA, there are two possibilities: