Groundless dispute

Company starts to increase traffic volume heavily. For some time they pay for the routes correctly.
After several months company begins to dispute a decent amount of traffic, something about 10% (it's actually the whole of supplier's income). They discuss nor CDR nor dispute's reasons.
After a while they start to demand CDR for different time periods, with short calls and without ones. They wouldn't give a definite answer, they just stalled, and demand new CDR for the current time period. They stop payments as well.
In the end supplier understand that he got swindled (he has about 2-3 non-payed bills) and close routes. Soon supplier receive a letter from his client with talking point that 50% of all calls for the last three months - FAS. He doesn't pay attention for the call duration being more than 4 minutes.
Usually customer provide nor documents nor proof.
He doesn't want to pay anything, even non-disputed sum.
Company management begins to hide answering nor e-mails nor phone calls.
They can sent you a letter with scrambled request to decrease amount of money in the invoices in compliance with FAS (but they do not specify what exactly calls with FAS they mean).
The best way to avoid such swindlers - close company immediately after receiving a dispute for the large amount of traffic till all reasons becomes clear enough.