<div class="contact-button link-phone {'path':'phone', 'id':'gtziy', 'id_raw': '243468578'} atClickTracking contact-a"
data-rel="phone">
    <i data-icon="phone"></i>
    <strong class="xx-large">HIDDEN TEXT HERE</strong>
    <span class="spoiler">SHOW</span>
</div>

IHtmlElement nodeToClick = (IHtmlElement)document.All.First(m =>
                    m.HasAttribute("class") &&
                    m.ClassList.Contains("contact-button") &&
                    m.HasAttribute("data-rel") &&
                    m.GetAttribute("data-rel") == "phone");

nodeToClick.DoClick();

<div class="contact-button link-phone {'path':'phone', 'id':'gtziy', 'id_raw': '243468578'} atClickTracking contact-a activated"
data-rel="phone">
    <i data-icon="phone"></i>
    <strong class="xx-large">TEXT HERE</strong>
    <span class="spoiler" style="display: none;">SHOW</span>
</div>

public static string GetHTML(string url)
{
        HttpWebRequest proxy_request = (HttpWebRequest)WebRequest.Create(url);

        proxy_request.Method = "GET";
        proxy_request.ContentType = "application/x-www-form-urlencoded";
        proxy_request.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.0.249.89 Safari/532.5";
        proxy_request.KeepAlive = true;

        HttpWebResponse resp = proxy_request.GetResponse() as HttpWebResponse;
        string html = "";
        using (StreamReader sr = new StreamReader(resp.GetResponseStream(), Encoding.UTF8))
        {
            html = sr.ReadToEnd();
            sr.Close();
        }

        resp.Close();

        html = html.Trim();

        return html;
    }