example of an html builder that htmlhelper extensions could return

PhoneHtmlBuilder.cs

public class PhoneHtmlBuilder<TModel>
{
    private HtmlHelper<TModel> _helper;
    private Expression<Func<TModel, string>> _phoneNumber;
    private Expression<Func<TModel, string>> _extension;

    public PhoneHtmlBuilder(HtmlHelper<TModel> helper)
    {
        _helper = helper;
    }

    public PhoneHtmlBuilder<TModel> For(Expression<Func<TModel, string>> property)
    {
        _phoneNumber = property;
        return this;
    }

    public PhoneHtmlBuilder<TModel> WithExtension(Expression<Func<TModel, string>> property)
    {
        _extension = property;
        return this;
    }

    public MvcHtmlString Render()
    {
        //probably register javascript to get rendered or generate some

        //i'm not even sure if this compiles, just getting a basic format of the idea down
        return MvcHtmlString.Create(
            _helper.LabelFor(_property) + 
            _helper.TextBoxFor(_property) +
            _withExtension ? _helper.TextBoxFor(_extension) : "" +
            _helper.ValidationMessageFor(_property));
    }

    public override string ToString()
    {
        return Render().ToString();
    }
}