Earlier quoted context omitted.
I think the correct approach is economic, not technical. Make a rule that, if you get a spoofed call, then your telco owes you $100, not subject to any arbitration clause or other contractual waiver. Give a way out to avoid actually killing the telcos: annual penalties are limited to a few percent of annual gross revenues. The telcos will find a technical solution real fast.
How would you prove the call was spoofed? Record all calls? Prove it with statistics? No telco will pay. This is a technical problem.
Or if they don't own it then it's on who's inserting the call into the system.
Telco systems are fun. And by fun I mean it's a complete mess. ISDN is hard and full of caveats (look at SIP for a taste of what it involves).