We do this with our web based CRM.
I push a registry entry to all our PCs that associates callto links (skype) to UCA via group policy.
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Classes\callto\shell\open\command]
@="\"C:\\Program Files (x86)\\Mitel\\Unified Communicator Advanced 4.1\\ucadialer.exe\" \"%1\""
From there, you can alter the webapp to format phone number links to look like "callto:+18005555555" (we do this in our CRM)
or
i also use a google chrome extension that uses a regex to change all phone numbers in webpages to callto links.
// Modified from: Skype Linkify (http://www.questar.it/blog/developer/skypelinkify.user.js)
// Match these patterns:
//  800-555-1212
//  (800) 555-1212
//  (800)555-1212
//  800-555-1212
//  800-555-1212
//  800 555 1212
//  800.555.1212
//  800/555/1212
//  8005551212
//  +1 (number)
//  + (international number)
//
// Link to "callto:<formatted number>" 
//
// ==UserScript==
// @name           Make Phone Numbers Callto: Links
// ==/UserScript==
//default prefix
const defaultPrefix= '+';
(function () {
 	const trackRegex = /(\+\d\d?)?[\-\s\/\.]?[\(]?(\d){2,4}[\)]?[\-\s\/\.]?\d\d\d[\-\s\/\.]?(\d){3,5}\b/ig;
	function trackUrl(t) {
		if (String(t).charAt(0)!= '+') t= defaultPrefix + String(t);
		return "callto:" + (String(t).replace(/[\-\s\/\(\)\.]/g, ""));
	}
    // tags to scan looking for un-hyperlinked urls
    var allowedParents = [
        "abbr", "acronym", "address", "applet", "b", "bdo", "big", "blockquote", "body", 
        "caption", "center", "cite", "code", "dd", "del", "div", "dfn", "dt", "em", 
        "fieldset", "font", "form", "h1", "h2", "h3", "h4", "h5", "h6", "i", "iframe",
        "ins", "kdb", "li", "nobr", "object", "pre", "p", "q", "samp", "small", "span", "strike", 
        "s", "strong", "sub", "sup", "td", "th", "tt", "u", "var"
        ];
    
    var xpath = "//text()[(parent::" + allowedParents.join(" or parent::") + ")" + "]";
    var candidates = document.evaluate(xpath, document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
    for (var cand = null, i = 0; (cand = candidates.snapshotItem(i)); i++) {
        if (trackRegex.test(cand.nodeValue)) {
            var span = document.createElement("span");
            var source = cand.nodeValue;
            
            cand.parentNode.replaceChild(span, cand);
            trackRegex.lastIndex = 0;
            for (var match = null, lastLastIndex = 0; (match = trackRegex.exec(source)); ) {
                span.appendChild(document.createTextNode(source.substring(lastLastIndex, match.index)));
                
                var a = document.createElement("a");
                a.setAttribute("href", trackUrl(match[0]));
                a.appendChild(document.createTextNode(match[0]));
                span.appendChild(a);
                lastLastIndex = trackRegex.lastIndex;
            }
            span.appendChild(document.createTextNode(source.substring(lastLastIndex)));
            span.normalize();
        }
    }
})();
This works great for us. I know its not exactly what you asked for, but should have the same results. Hope it helps.