Follow up emails are good, calls can be a bother. Some websites state that they do not want be contacted via phone about jobs. So becareful before calling.
What I suggest for follow up emails is that before you send out the first one, have a piece ready to go up on your site but hold off from posting it. Of course make sure your portfolio is strong enough and doesn't need the peice. Then a week later post it, and send out follow up emails if any fish haven't bitten. "Hey I just updated my site and I wanted to make sure you where aware bla bla bla" That way they have something new to look at and can gage the time it took you to do something.
About your particular portfolio.
I see zero finished game art. This will hurt you. Knowing Zbrush is nice but what is critical is knowing the 3D app the company uses. Normally 3dsMax or Maya.
You need to demonstrate that you can successfully complete all the tasks they will ask of you. If you are applying for a modeling position, that means showing low poly modeling techniques, carefully unwrapped UV layouts, well painted textures and possibly a rigged/animated character. Notice that ZBrush doesn't fall into too many of those catagories and that the work you have posted is untextured, possibly unwrapped, unrigged and even the low polys are a bit high poly.
It looks like you're going for character modeling positions, which are pretty hard to come by and the competition is feierce. It seems that is mostly what schools focus on for some reason. Which is kind of wierd when the bulk of the work is often done by enviroment artists.
In short, finished game art, fully textured and if you have the know how and the time rigging and animating is great. It also helps to have examples of your models made into custom player models in popular games that support those types of things. id games, Epic games, Valve games ect...
I'd say if a place was looking for a zbrush artist you could apply with what you have but not much more outside of that, until you can demonstrate you have the other skills it takes to make game content.
Personally I think there are WAY too many character artists portfolios out there and not enough Environmental Artist portfolios. I would ballance it out and work on Environment props for a while, your chances of getting in as an enviro artist are greater because the competition is less feirce and there are more jobs available. If your heart is for doing character art, then being on the inside already will help you get there.