Number Porting is the process for migrating a WhatsApp Business API that is currently in another BSP to 360dialog.
If you are not sure that the phone number is registered in a WhatsApp Business API account, there is a simple way to discover it.
When you message this phone number, if the Call icon
appears in the top right of the app, this is a WhatsApp Business App account.
This means that to register this account in the WhatsApp Business API you will need to use either Embedded Signup or Classic Signup, and not Number Porting signup.