Why do they have to rely on some Ukrainian official?
If these are recordings of a face to face meeting in Ukraine made by the Burisma exec then there would be no way to eavesdrop ...no phones involved.
If these are recordings of calls made within Ukraine during Biden’s visit then it is unlikely we would be eavesdropping like we could on international calls.
And since it is the Bidens and not Trump on the phone there wouldn’t be a reason for the leftwing nuts in our government to eavesdrop, and every reason not to.