Trump tweets Tuesday the April call is "more important".
The inquiry comes three years into Trump's unconventional presidency and as Democrats and Trump's Republicans are gearing up for a brutal election battle.
The Bidens have said that Trump's allegations were unfounded.
A National Security Council official assigned to the White House, Lieutenant Colonel Alexander Vindman, has testified that the first Trump-Zelenskiy call on April 21 "was actually a very good call".
Biden is a leading contender for the Democratic nomination to face the Republican president in the 2020 election.
Both raised concerns about an apparent effort by President Trump's personal attorney Rudy Giuliani and other USA diplomats to pressure Ukraine to announce investigations into the president's political rivals.
Bengals will evaluate Colin Kaepernick's National Football League workout
Well, at least as well as it can aside from live game action. "I'll be curious to see how he does". Sources previously told ESPN that Kaepernick and his reps were alerted to the workout at 10 a.m.
The first public hearings in the inquiry are scheduled for Wednesday and Friday. None were removed from office, although Richard Nixon resigned as he faced nearly certain impeachment in 1974 over the Watergate scandal.
"I think Ukraine is kind of a victim in this case because I believe they were pushed to do something that is not in the real Ukrainian interest", said Igor Yarmak, 63, a tech professional in New York City who has voted Republican in the past but dislikes Trump.
She said that all U.S. national security departments and agencies agreed that the military aid was essential despite the White House's hesitation to release it.
Witnesses in the probe so far, in hundreds of pages of testimony transcripts released in recent days, have highlighted serious concerns raised by senior State Department and other officials over Trump and his administration's handling of Ukraine.
"All of the senior leaders of the USA national security departments and agencies were all unified in their - in their view that this assistance was essential", Cooper said, according to the transcript. The Intelligence Committee will stream the video on YouTube. The proceedings will begin with statements from House Intelligence Committee Chairman Adam Schiff, and the Committee's ranking Republican, Devin Nunes.