Strictly, being 'downsized' means being fired because the company is laying off workers for economic reasons. Losing your job for misbehaviour is better called "getting the sack" or simply "being fired".
I would agree with you if the sentence read as you've written.I, too, question the use of "downsized," but I would like to leave that and comment on the sentence as a whole.
While I find it clear enough, the sentence did leave me a bit confused. I had to read the sentence again to make sure I had understood. I believe the problem is here: "...for an innocent mistake by calling..." This sounds awkward to me.
I would agree with you if the sentence read as you've written.
But it actually says, "... for an innocent mistake he made by calling..."
He was fired for an innocent mistake, which he made by calling his wife ...