In the call record the White House released, which President Trump has said is an exact transcript of his conversation, President Trump mentions Biden and his son repeatedly to Zelensky. So that’s first hand info.
Then, even though Trump has never mentioned Burisma or any company on the call, Zelensky makes the connection between the Bidens and "the company that you mentioned in this issue" and says his next prosecutor general will look into it. whitehouse.gov/wp-content/upl… 
This evidence, in addition to fully corroborating Taylor's testimony and making @RepLeeZeldin look like a fool, suggests the existence of additional, as yet undisclosed communications between Trump and Zelensky regarding both the Bidens and Burisma.
