Complex sentence: TelstraClear customers, mostly corporates, used to be able to get mobile numbers under this prefix; these customers were billed by TelstraClear instead of being billed directly by Vodafone.
Split into simpler sentences: TelstraClear customers, mostly corporates, used to be able to get mobile numbers under this prefix. These customers, however, would be billed by TelstraClear instead of being billed directly by Vodafone.