What if they had transcripts of the calls, but not the audio?
To have transcripts would require actual wiretaps, which would have to be approved by a judge to be legal. If they went to a judge, they would have to specify exactly who they wanted to tap, and provide evidence of a reasonable suspicion that each person included in the wiretap request was involved in a crime. No judge is going to sign off on a wiretap order for 20 or more journalists, especially if the communications being tapped involve members of Congress.
So if they had transcripts that would be prima facie evidence that they had violated federal wiretapping laws and subject to prosecution and imprisonment.