I'll have to do a quick search for links, but President Zelensky has publicly stated that the U.S. Transcript is accurate as to the discussion with the call. It should be noted that even if a Ukrainian transcript does exists, it will not be a 1-1 accuracy check as not only do the Ukrainians not use the same language, but they don't even use the same alphabet. Diplomatic translators may also not make direct translations in some circumstances because of how languages work.
For example, let's say that in a hypothetical call between the U.S. and Ukrainian Presidents (generic Presidents, not the specific ones), the Ukrainian President says "I feel like I received a Cossack Manifesto". The U.S. translator might tell the U.S. President that the Ukrainian "feels like he was insulted" instead of the direct English translation. The U.S. President would likely not get that the Ukrainian President is referencing a particular famous event in Russo-Ukrainian history and the famous painting depicting it (Reply of the Zaporozhian Cossacks to Sultan Mehmed IV of the Ottoman Empire aka "Cossacks of Saporog Are Drafting a Manifesto"). While it's perfectly possible to explain this to the President, it's not needed to understand the translation. The same will happen in the reverse, though since we're all decent English speakers here, it's easier to come up with a non-U.S. or Western reference.
In diplomacy, it is rare for foreign leaders to speak to each other without translators, even if the specific leader is an accomplished speaker of the language. The reason is because these discussions can lead to major disasters if idioms are not fully understood -- sometimes, even if the same language is spoken. For example, in the Korean war, there was one incident where the British lost a battle because the U.S. didn't send any reinforcements to the Brits. This was because when the Americans asked for a Sitrep, the British radio operator told them things were "a bit sticky" which the U.S. thought was the British brushing off the trouble. However, the British understood it as, what in the U.S. Military at least, would be a call that the situation was FUBAR (Fouled Up Beyond All Reason -- though, this term originated in the Navy, where they used a different F-word for "Fouled" that I can't repeat on this website).
A translator specializing in the language has much more time to understand idioms in vogue in a language and can find an appropriate expression to relay the idiomatic meaning in a way that makes sense, and even then, there can be some translation failure. For example, the Daily Show once interviewed a Japanese translator with the U.S. State department where she revealed it was rather difficult to explain why Trump's comments on the infamous Billy Bush tape ("Grab her by the...") were shocking when translated into Japanese. While there are certainly Japanese words for the body part discussed, the Japanese do not use those words in an insulting or overly crass manner, and they had difficulty understanding what was so off putting by the comments when translated (or that the term used had a negative connotation at all, since again, there equivalent terms aren't used in that way). Though bizarrely, Japan and English do have an unusual distinction in that both languages word for "dream" (Japanese: Yuma) have exactly the same meaning when used in context (both the phenomena that occurs while sleeping and the idea of a high or lofty goal or ambition that one wishes to obtain.) Given the fact that the languages have no common roots shared between them (pre-1945, a lot of English Loan words entered the Japanese Language after the U.S. Occupation).
