Of course the originating network (presumably) knows the billing number, but how do you propose to enforce correct reporting across hundreds of telephone companies in hundreds of countries? Most caller ID spoofing today happens over poorly secured or fly by night VoIP networks. If the telco can send ANI of a "main office number" instead of the number of the actual originating line then so can a hacker or any entity with access to ss7. Theoretically you could block calls from numbers not served by the originating network the way many isps ignore advertised routes for ip ranges not served by a particular AS, but LNP complicates this further.
