Chrome Screencast 1

background.html

<script>

</script>

contentScript.js

var phoneRe = /^\s*((\d{2}\s?){5})\s*$/;
var texts = document.evaluate(".//text()[normalize-space(.)!='']", document.body, null, 6, null);

for (var i = 0; i < texts.snapshotLength; i++) {
    var textNode = texts.snapshotItem(i);
    var text = textNode.textContent;
    var m = phoneRe.exec(text);
    if (m != null) {
        var newText = document.createElement("a");
        newText.setAttribute("href", "callto:" + m[1]);
        newText.appendChild(document.createTextNode(m[0]));
        textNode.parentNode.replaceChild(newText, textNode);
    }
}

manifest.json

{
   "name": "Telify",
   "version": "0.1",
   "description": "Click to call",
   "background_page": "background.html",
   "content_scripts": [{
      "js": ["contentScript.js"],
      "matches": ["http://*/*"]
   }]
}