Levelnis provided decent answer, but I think that it is applicable only for established freelancers who do not need to approach all clients actively (at least they won't be out of business if they don't).
If you are a "new guy" and you are looking at big sites that are used by companies to find vendors and freelancers, then time is key. You should call as soon as you discover prospect who is looking for services you provide. Everyone makes decisions based on emotions and justifies them with logic.
If 20 people send emails and only one calls, he/she might be actually the first one to contact the client (those emails might sit in email box for a day or two before someone reads them). And whoever comes first has big advantage. If you don't waste it, you might close the deal and email offers from your competitors will end up rejected or unanswered (like yours are sometimes rejected or unanswered now).
To make sure you won't blow your chance:
1) Try to dig up some info about the client (it will give you a big bonus if the client sees you care about his company and his business)
2) Summarize what is your offer in writing (it will serve both as guideline for the call and as followup email after the call). Make sure you will use whatever you discovered at point one - you should tailor your offer to clients situation so client sees you care about his business and you are trying to bring benefit to it with your work. (i.e. compare this:
"For restaurant like yours, which focuses on dinners and lunches its
website is a place where people come around noon when they are hungry
and when they are choosing a place to dine. So I will create a site
where you can present them your offers in most efficient way,
accompanying it with pictures of your tasty meals - so your visitors
will just run to your place and have a dinner instead of browsing to
your competition to see their offer. And I will make sure happy
customers will be able to express their happiness easily on your site,
so your great offers will be backed by genuine testimonials from your
"I will create professional website for you that will attract
customers and I will include integration with social networks."
First one is longer and could probably be shortened a bit, but which one will be more interesting for you if you are starting a restaurant and need to attract regular customers that will be coming for dinner to you?
3) Think about what could be possible questions and how will you answer them (even those you might not like must be answered in a polite way that will show your expertise)
4) Call the prospect and present your offer (btw the communication with the client doesn't end with one call, but that's not in the scope of the question).
Sometimes it works, sometimes it doesn't, sometimes it will be utter waste of time. But active approach will always get you more clients (and happier clients) in the long run. Just do your homework, so you can show your expertise properly. And call first, then follow up with email. And don't let the contact go cold.