Contractor poured new concrete for new garage. Today I noticed that vertical foundation has unfilled areas. I am not happy. Should I be concern? He did come back to fill them up with cement but I still concern.

Hard to tell if you are talking about honeycombing (which coms from dry mix, or not vibrating enough etc) and/or whether these "unfilled areas" will be visible or backfilled against.
If it is minor honeycombing and repairs were done the day with a proper mortar mix-and with a proper cure after being placed, maybe not so bad.
If you are describing actual large voids that's another story.