> What kind of measures are possible to prevent fraudulent calls when the caller is your anonymous customer?

Presumably some fairly basic heuristics would be sufficient. Robocalling isn't economically viable if you only get a few calls per subscription. You need to place (I assume) at least thousands of calls per day per subscription for it to even begin to make sense. Any account doing that is going to be blindingly obvious provided you have even 30 minutes worth of logs.

I can already walk into Walmart and purchase a cheap prepaid device with cash. That's pretty close to anonymous.