In 2014, Firtash was arrested in Vienna and accused of racketeering, via a titanium mining racket involving Boeing. In seeking Firtash’s extradition to the US, authorities revealed in court that they had wiretapped Firtash’s telephone conversations. Source They later revealed that they possess thousands of intercepted communications. Source
It would be interesting to learn whether Firtash’s intercepted communications included any calls with Manafort.