‘Called’ implies by subpoena. Rather he was deliberately invited by the House. They should have called him and forced it to the Judiciary to resolve. While not likely, SCOTUS may have declined the case letting the subpoena stand. I would have enjoyed having SCOTUS clarify the validity of the initial subpoenas and Executive Privilege, if claimed. My gut says the House would have won on the latter.
I think they absolutely would have won. I can't figure out how the WH could make an executive privilege claim stand after they already released the call summary, Trump has tweeted endlessly over it, Mulvaney spoke to the press about it, etc. The call itself is hardly private anymore. Bolton testifying would simply be having someone who was in the room for the call provide more context.