[H]ave [Congressional Democrats] called Assistant Attorney General Brian Benczkowski, the head of the DOJ’s Criminal Division, who according to this article made the final decision not to open a criminal investigation?
The Senate Judiciary Democrats didn't "call", they "called on" by sending a letter.
Senate Judiciary Democrats to Assistant AG: Why Didn’t DOJ Investigate President’s Actions?, October 10, 2019.
Senate Judiciary Committee Ranking Member Dianne Feinstein (D-Calif.) and all committee Democrats today called on the Justice Department to explain why it decided to not investigate President Trump’s solicitation of foreign interference in the 2020 election during a phone call with the Ukrainian president.
“On its face, the White House memorandum of the July 25 call raises significant questions, including what was done following the call to follow through on the president’s requests,” the senators wrote. “Since this matter involves allegations about the president’s conduct, and given this president’s stated belief that he has the ‘absolute right to do what I want to do with the Justice Department,’ it is important to ensure that the department has acted in an objective, independent manner in declining even to investigate this matter.”
The questions asked:
What Department components and individuals were consulted prior to the Department’s concluding that “no further action was warranted”?
What evidence did the Department consider beyond the summary of the July 25 call, if any?
Did the Department refer the matter to the FEC as required?
Did the Department consider whether President Trump or any other individuals, including but not limited to other Executive Branch officials and Rudy Giuliani, violated any statutes other than 52 U.S.C. § 30121 in connection with the conduct described in the whistleblower complaint?
What role did OLC play in the Department’s decision that “there was no campaign finance violation” and “no further action was warranted”?
Did OLC evaluate whether the whistleblower complaint implicated potential criminal violations before OLC concluded that the complaint should be referred to the Criminal Division rather than forwarded to Congress?
From the above questions, it seems clear that the Senate Judiciary Democrats are trying to collect background information to further narrow their oversight investigation. A response was requested by October 24, 2019. I do not know if a response was given.
And have they called Attorney General Bill Barr to testify about the extent of his involvement in the decision-making process?
Not that I have found.
One of the questions, as noted above, in the letter to Assistant Attorney General Brian Benczkowski was: What Department components and individuals were consulted prior to the Department’s concluding that “no further action was warranted”?
If the response to the letter mentioned Attorney General (AG) Barr, that would give some impetus to inquire further and, perhaps, testify; but, for now, the Senate Judiciary Democrats did "call on" AG Barr to recuse himself.
Senate Judiciary Democrats to AG Barr: Recuse Yourself from Ukraine Matters, Octtober 24, 2019.
All 10 Democrats on the Senate Judiciary Committee have called on Attorney General William Barr to recuse himself from matters related to Ukraine because of concerns about his role in President Trump’s efforts to damage a political opponent and undermine the Russia investigation.
“Impartial enforcement of the law is essential to give the American public confidence in the Justice Department’s work,” the senators wrote. “Your personal connection to these matters creates the appearance of a conflict of interest and gives rise to questions about whether the Department is being used to advance the President’s personal interests.”
In a separate letter, the senators asked Assistant Attorney General Lee Lofthus, the department’s ethics officer, whether Attorney General Barr has been advised on this matter by department ethics officials.