Ok, so assuming this witness exists, Z didn't walk to the phone booth, he drove. This implies that he didn't live super close. If this is the case, then for me it makes the time gap even more intriguing. Z using the time to go home, and then walking over to make the call, would be a very plausible reason for the gap of time. I would think that if he did indeed go home following the attack, and lived close enough to the phone booth to walk there, he would choose to walk not drive. Obviously this is built on assumptions, but food for thought.