Washington (CNN) Phone records released this week as part of the House Democrats' impeachment report revealed a series of calls from President Donald Trump's personal attorney, Rudy Giuliani, to an "unidentified" number designated simply as "-1" that has investigators wondering who it is.

The phone company logs, according to the report, also show Giuliani made several calls to numbers associated with the White House and the Office of Management and Budget during that same time frame, further fueling speculation about who he was talking to and for what purpose.

While the substance of these conversations remains unclear, phone records reveal a flurry of calls involving Giuliani in April that could be relevant in the ongoing impeachment inquiry into Trump's dealings with Ukraine, according to the report by Democrats on the House Intelligence Committee.

The calls also came at a time when Giuliani was helping deal with the fallout from special counsel Robert Mueller's report on Russian interference in the 2016 presidential election after a redacted version was released that same month, one White House official noted Wednesday.

Now, months after they took place, those calls are at the center of a mystery that could have significant implications on the impeachment process.

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