I spent a lot of time working on this exact problem at a "Big Tech Retailer". 6 different teams had worked on it before we did, and all had given up. This is actually a very difficult problem that is at the intersection of two other very big and familiar problems... spam phone calls and bots on the internet.
Spam phone calls... the global phone system is a network of relays. No telecom provider connects everyone on the planet together. To call our grandmother in Russia, we may have to go through Verizon, Deutsche Telecom, MTS, and ~five different smaller, regional telecom providers. The first telecom provider will request the second to complete the call, will trust they do this, and will accept the price they charge upon which they'll add their own costs. This occurs recursively until the phone call has been connected and completed. This implicit trust enables fraudulent actors to get into the circle of trust. Verizon may trust Deutsche, Deutsche may trust MTS, and MTS may trust a smaller telecom provider who in turn trusts a spam caller. This enables you to get spam calls. Telecom providers themselves don't know all the callers on the global telecom network and don't really know how much people will be charged. There is no global government to legislate across all telecoms.
Bots on the internet... the internet as a whole doesn't have a firm sense of identity. It's just a network protocol routing packets to ip addresses. In the past, these ip addresses were mostly human beings. In the current time, the majority of the participants on the internet are bots/computer programs. A website like "Big Tech Retailer" has >90% of all traffic from computer programs. Elon Musk was probably right that Twitter is full of bots, because the entire internet is swimming with bots. They can be incredibly difficult to detect because AI blurs humans with bots.
This toll fraud problem is that bots we struggle to detect place phone messages to phone numbers we struggle to identify. This ends up costing a huge and growing amount of money. You cannot truly solve the problem without solving the two underlying problems of bots on the internet and spam calls. Solutions to those problems may require rethinking and rebuilding the entire communication system we've built our lives around.
Nonetheless, we can greatly reduce the effect of this problem. At "Big Tech Retailer", myself and two others we were able to reduce the cost to a small percentage of what it was. After that point, the business sort of stopped caring because the fraud cost less than the staff. There were perhaps five techniques that were most helpful, all of which were contemporary fraud fighting/bot fighting/security techniques.
If you're a startup facing this problem, I can help give you some guidance. Twilio will probably see this post and start working on a solution, but that may take a long time. There are easy things you can do to mitigate the problem right now. You can contact me at manrajt@gmail.com.