The phone is actually a font icon fron this set
http://fortawesome.github.io/Font-Awesome/icons/These are already installed in the cart files, but you can see the icons here if you wish to use a different one.
The line you need to change is in :
/storefront/view/default_html5/template/pages/product/product.tpl
On my editor is it line 185
<li><a href="#" class="call_to_order"><i class="icon-phone-sign"></i> <?php echo $text_call_to_order; ?></a></li>
It is the icon-phone-sign that you will change
Lee