Depends what you mean by "premium rate." Every number costs money to call in Twilio. Some numbers cost more, in lots of these frauds numbers in ordinary ranges are used (Is a rural number in Chile that costs $0.20/minute to call premium rate/fraud? Because that's what it looks like a lot of the time. How about $0.05 a minute in Austria?). IRSF, the industry term for this kind of fraud causes billions in losses a year and there is no easy answer but Twilio should probably have more infrastructure in place to reduce massive surprise bills.
Block any number that costs more than 25th percentile would be a start and so on... I can come up with plenty of heuristics that would be better than nothing.
