But it's his work with Volker on Ukraine that has embroiled him in this controversy and made him a key witness.
In a statement, Sondland's lawyer says he's "profoundly disappointed" and "stands ready to answer the committee's questions fully and truthfully" if the State Department and Congress work this out.
According to text messages released by House committee leaders last week, Sondland was heavily involved in contacts with Ukrainian President Volodymyr Zelenskiy as he sought a meeting with Trump, and Ukrainian officials expressed concern at the administration's decision to block almost $400 million in U.S. military assistance for Kiev.
Sondland, 62, is one the diplomats involved in numerous text messages released by the committees last week that show discussions about Trump's July phone call with Zelensky, which was the subject of at least one whistle-blower report.
Sondland's absence raised questions about whether other witnesses called by the committee would appear.
House Intelligence Committee chairman Adam Schiff said investigators have learned that Sondland has text messages or emails on a personal device that are "deeply relevant" to the probe, but that State is withholding them. He noted that while he would "love" for Sondland to testify, it would be "before a totally compromised kangaroo court".
Sondland's attorney released a statement explaining that the State Department had "directed" Sondland to skip his scheduled, voluntary testimony before three congressional committees pursuing an impeachment inquiry against President Donald Trump.
The White House letter spelled out Trump's evolving strategy of stonewalling lawmakers' demands for witnesses and documents, while questioning the legality of the inquiry. The State Department did not respond to requests for comment on why Sondland had been blocked from speaking to lawmakers just hours before his scheduled appearance.
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Administration officials discussed whether to block Sondland's testimony late into Monday night, CNN reported.
In a series of text messages with Volker, the two senior diplomats appear to be drafting an announcement for Zelenskiy to make regarding an investigation into Democratic presidential candidate and former Vice President Joe Biden, his son Hunter, and Hunter's business dealing in Ukraine, as well as allegations that Ukrainian officials leaked damaging information about Trump's 2016 campaign manager Paul Manafort, who has since gone to prison, in part, for his business dealings in Kyiv.
A complaint filed against Trump based on the call was filed by a person who has not been identified publicly. In those texts, Sondland denied that there were any "quid pro quo's".
After Taylor expressed concern in a September 1 exchange that withholding U.S. military funds in exchange for an investigation was "crazy", Sondland responded, "the president has been crystal clear no quid pro quo's of any kind".
The Democratic-led inquiry is trying to establish whether the Republican president withheld almost $400m (£327m) in aid to nudge Ukraine's president into launching an inquiry into Mr Biden, whose son, Hunter Biden, was on the board of a Ukrainian energy company. He said the panel is probing whether Trump solicited foreign help from Ukraine for his 2020 reelection, whether a never-realized White House meeting between Ukrainian President Volodymyr Zelenskiy and Trump was conditioned on the country conducting investigations, whether US military assistance to Ukraine was conditioned on those investigations and whether the administration has obstructed justice.
In another exchange, Bill Taylor, the senior U.S. diplomat in Ukraine, questions whether conditions are being attached to United States military aid and a White House invitation for Ukraine's president.
In one of the texts, Mr Sondland said Mr Trump "really wants the deliverable".
"I suggest we stop the back and forth by text,"Mr Sondland added".
Last week, Trump downplayed the text messages, saying Friday he didn't "even know most of these ambassadors".