What you are describing is already done by carriers in countries with high sim box usage. (Basically, it costs ~$0.01/min to make a local call in Nigeria but $0.25/min to make an international call to Nigeria so people there set up SIM boxes and Asterisk to terminate calls locally and profit the difference between these rates. The reason it costs $0.25/min to call the official way is due to many governments taxing incoming international calls because they see it as an easy way to raise revenue at other people's expense). But anyways, because governments don't like this kind of arbitrage, they force carriers to add detection mechanisms. So they check for high ratio of outbound to incoming calls, high amount of distinct phone numbers called, 24/7 usage patterns, etc. Except Africa is still losing a few billion dollars a year to this kind of toll bypass because it is still massively profitable (see https://en.antrax.mobi/request-pricing/ for example) and these changes just require them to rotate sim cards slightly more often. Essentially what I am saying is that unless you can reduce the fraudsters' margin by a substantial amount you are wasting your time.