Currently, it looks like I have it configured to allow all destinations that are $0.40/minute or less with voip.ms's "value" routing. This is enough to call all numbers in ~120 countries and some numbers (usually landlines) in another ~70.

And there are occasionally people who have trouble with the concept of "free 5 minute call" (that it hangs up after) not meaning "just keep dialing back for an hour or two", even with a long line behind them. I can't ban specific outgoing numbers with voip.ms, but once I ended up just disabling all calls to the country that a problematic user kept coming back and repeatedly redialing.