Wow. That's scary--the thing that freaked me most was the caller, though. Mysterious phone calls have always been very unnerving for me.
This is good. I love the atmosphere and the tone, though I think a little more description would make it even scarier. Describe his emotions, for one--foreboding when the phone rings, for example. Don't just tell us he said "This is sick" when watching the woman on the screen, make us feel it.
I think glossing over his days at work made the story seem a little odd. It broke the mood, maybe. Maybe add little things during the day when he's not at home--when he remembers that call and the picture on his screen and feels a little freaked out--that way you keep the mood going and it doesn't seem like you're racing to get to the good parts of the story.
Also--and this is just a thought--I think you could add even more chills if, at the end as the man approaches Zander with the knife, you had the man's voice also echoing from Zander's computer speakers. The camera must have a mic or something, since Zander could hear him before, after all. Unless the mic was actually in the room where he had the woman ...
Thanks for writing,
Ashe