Most of the President's calls are subject to being classified. This particular call had to be de-classified to be released. Trump also had to get Zellenskyy's permission to declassify the conversation and release the transcript. Included in this call was a discussion of the U.S.'s support for Ukraine vs. Russia, criticism of Angela Merkel/Germany and the EU for not supporting Ukraine enough, and the U.S. criticizing and replacing its Ambassador to Ukraine. Zellenskyy discussed plans to investigate corruption in his country and hire a state prosecutor that supports his administration. Both the U.S. and Ukraine would normally agree that such a conversation should be classified. Neither Trump nor Zellenskyy want to conduct their bi-lateral relations through the New York Times. If these conversations were public, then no one would dare to say anything sensitive. They'd just talk about the Mets.
The question here is whether Trump and Zellenskyy's discussion on corruption was legitimate, or whether it amounted to a form of bribery or blackmail on behalf of Trump's personal political campaign. Obviously, Trump denies this. Given that there is a history of corruption in Ukraine, given that the DOJ is actively investigating corruption on the part of U.S. officials in Ukraine, and given the admitted history of Joe and Hunter Biden, an argument can easily be made that discussing corruption and mentioning the Bidens is both legitimate and justified, especially when Zellenskyy was the one who raised the subject.
