Use font awesome icon as css content

That's the wrong way to use it, open the font awesome stylesheet, go to the class of the font you want to use say fa-phone and then copy the content property under that class with the entity and than use it like

a:before {
   font-family: FontAwesome;
   content: "\f095";
}

Demo

Just make sure that if you are looking to target a specific a tag, than consider using a class instead to make it more specific like

a.class_name:before { 
    font-family: FontAwesome;
    content: "\f095";
}

Using the way above will stick the icon with the remaining text of yours, so if you want to have a bit of space between the two of the, make it display: inline-block; and use some padding-right

a:before {
   font-family: FontAwesome;
   content: "\f095";
   display: inline-block;
   padding-right: 3px;
   vertical-align: middle;
}

Extending this answer further, since many might be having a requirement to change an icon on hover, so for that, we can write a separate selector and rules for :hover action

a:hover:before {
 content: "\f099"; /* code of the icon you want to change on hover */
}

Demo

Now in the above example, icon nudges because of the different size and you surely don't want that, so you can set a fixed width on the base declaration like

a:before {
   /* Other properties here, look in the above code snippets */
   width: 12px; /* add some desired width here to prevent nudge */ 
}

Demo