But that someone - who just happened to be an Upcoming Witness for the Jan. 6 panel - didn't pick up the phone and instead reported Trump's attempt to reach him/her to their lawyer.

The lawyer contacted and informed the Jan. 6 Committee.

The committee contacted the Department of Justice which is now investigating the attempted phone call from the former president.

The question is "who is this upcoming witness" that Trump was so worried about he took a stupid risk and personally called to intimidate?

The most logical guess is Mark Meadows.

First, Meadows hasn't totally cooperated with the J6 panel. 

He was cooperating at one point, then Trump told him to clam up. And he did.

Secondly, the last few J6 hearings have dropped some bombs about Trump from close White House aides that paint a picture of Meadows complicity in not stopping Trump's brazen plans to overthrow the election.

Meadow's former assistant Cassidy Hutchinson provided damning testimony that revealed how afraid he was to deal with Trump's lunacy.

You can bet that the DOJ has been exploring Meadow's actions closely, especially after Hutchinson's testimony. They may have enough on him to offer a deal - immunity from charges (or reduced sentence) in exchange for first-hand quotes from Trump directly linking him to his co-conspirators in Congress and outside lawmakers and activists working with him to stop the electoral process.

Finally, it's not likely Meadows would take a fall and go to prison for Trump, regardless of how afraid he is of him. With new information flooding out of the J6 panel weekly the odds of Meadows taking flak from witnesses increases.

In summary, the DOJ would have the smoking gun it needs to indict and convict Trump of treason and other serious charges.