Simbox International incoming traffic

Our telecom is very suspicious nowadays that simbox is working within our GSM network. Because our International incoming revenue has been decreased sharply due to unknown reasons. Our Telecom takes 30 cent for each incoming international call. Therefore there should be a cheaper route which the abroad telecoms are routing our traffic to. Also how did that Telecom operators abroad know or how are they informed that there is a cheaper way to route our traffic. And how did that simbox owners advertise their cheaper routes? of course they take a cheaper rate may be about 10-20 cents per minute. Is there a website or a online stock market which advertise all routes to different operates with different rates? I hope somebody will provide this info.

Kind Regards

Abdel