Defendants don't call any witnesses at all when the Crown hasn't met their burden of proof. They quit while they're ahead.Don't be silly. Of course most defendants prevent witnesses who have exculpatory evidence from testifying. The call was perfect so of course he doesn't want any witnesses to tell anyone how perfect it was.
Of course this isn't a court trial, so there can be, in theory, political consequences to choosing not to call witnesses. However, I don't think that the average voter, and more importantly, independant voters, care enough about even the fundamental charges against Trump for such minutiae as who he tried to call, or didn't try to call, to affect their political judgements.