 <p>I have now switched from <code>wsDualHttpBinding</code> to <code>NetTcpBinding</code> and for some reason, everything is working fine. </p> <p>I have used <a href="http://www.singingeels.com/Articles/Duplex_WCF_Services_Hosted_in_IIS_Using_NetTcp.aspx" rel="nofollow noreferrer">this article</a> to help me set up hosting on IIS, and thankully everything is working as expected, with callbacks.</p>