He also described confronting Gordon D. Sondland, the United States ambassador to the European Union, after the envoy pressed Ukrainian officials to help the Trump administration by investigating the Biden family. The colonel said he acted out of a “sense of duty,” and emphasized his military service in his remarks. “I am a patriot,” he said, “and it is my sacred duty and honor to advance and defend our country irrespective of party or politics.”

As he spoke, House leaders were preparing for what was expected to be significant new private testimony from current and former White House officials in the coming days. On Wednesday, they will hear from two Ukraine experts who advised Kurt D. Volker, the former United States special envoy to the country. On Thursday, Timothy Morrison, the National Security Council’s Russia and Europe director, is scheduled to testify. And on Friday, investigators have called Robert Blair, a top national security adviser to Mick Mulvaney, the acting White House chief of staff.

There is no recording of the July 25 call by the American side. The White House uses note-takers listening in on the call as well as voice recognition software to create a rough transcript that is a close approximation of the call. But names and technical terms are frequently missed by the software, according to people familiar with the matter.

After the call, Colonel Vindman was given a hard copy of the rough transcript to make updates and corrections, according to a person familiar with the matter. Colonel Vindman went through the transcript, made changes and gave his written edits to his boss, Mr. Morrison, according to the person.

But after the call, Colonel Vindman went with his brother, a lawyer on the National Security Council staff, to see John A. Eisenberg, the council’s legal adviser, to raise his concerns about the conversation.

Colonel Vindman declined to detail to investigators his discussions with Mr. Eisenberg, citing attorney-client privilege, according to two of the people familiar with the testimony.

One explanation for why Colonel Vindman’s changes were not made could be that the transcript had been quickly placed into a highly secure computer system, the N.S.C. Intelligence Collaboration Environment, or NICE system, making it more difficult to alter.