I am being billed for international calls I did not make, what should I do?

I just got a bill for like 5000 dlls for makes that my family members (two minors) and I made to El Salvador and Mexico. I make one international call and its not to those numbers. I called my phone carrier and they are conducting a study to see if its fraud and correct the chargers. But what can I do, I want to make the people suffer legal action for this fraud, I want to find out who and how. What can I do? How do I prove it wasnt me making the phone calls.