Provider billing add-on is an additional functionality which allows reversed billing – it counts provider’s balance which increases when admin or reseller makes a call through it.
Provider has balance which increases when call goes over this Provider. E.g. Provider receives money for the calls over him.
If Provider is assigned to the User then such Provider can make calls. When Provider (which is assigned to the User) makes calls, such calls goes over other Providers and User’s (to which Provider is assigned) balance is decreased.