Are you up to it, @realDonaldTrump?
There is no such thing as testifying by tweet.
#1: The call was perfect
#17: Zelensky said there was nothing wrong
#21: Trump did more for Ukraine than Obama
(I need to make a card with more squares)
If the House issues a subpoena, he can refuse, but that's more obstruction.
He can take the Fifth, but the President of the U.S. refusing to testify because of criminal liability wouldn't play well in an impeachment trial.