Smaller telcos restrict the outbound callerID to only CallerIDs (DIDs) that you own. This would be very difficult to enforce for a wholesale carrier - where their customer is an aggregate of thousands of customers.
Billing in telcos is important and has all the details you need. This is either information of who placed the call (if they're your customer) or which carrier the call came from (if they aren't). The only problem right now is that there's no reason for responsible behaviour. As soon as carriers are forced/want to say "stop sending us spam calls, or we stop taking your traffic" to both customers and other carriers, this will get sorted.
But changing IDs is already illegal, so the FCC could decide to chase it down if enough people complain.