How is the Premium offer billed?

The Premium offer is not linked to your Onoff number, but to your account.

It is therefore naturally billed separately from any other paid service.

This means that if you delete your number, or change your number, the Premium package will remain active on your account.

If you wish to cancel the Premium offer, you must do so independently of your Onoff number(s).
(Check "How to cancel my Premium offer ?")