AT T Posts \$7.1 Bln Loss on Writedown  WASHINGTON (Reuters) - AT T Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=T.N target=/stocks/quickinfo/fullquote"&gt;T.N&lt;/A&gt;, the No. 3 U.S.  telephone company, on Thursday reported a \$7.1 billion  third-quarter loss due to \$12.5 billion in charges for cutting  asset values and jobs after its retreat from consumer services.