AT&T "has a ton of debt," he said Wednesday on CNBC's "Squawk Box." "This is a very mature business. Revenues have been declining." Shorting is a trading strategy that involves selling a borrowed asset with a view that it will drop in value and the asset can be bought back later at a lower price. In his presentation for the Sohn Tel Aviv conference Wednesday, Westhus said AT&T added $125 billion in additional debt to its balance sheet since 2007. He noted that the wireless business is facing more competition and has deteriorating growth trends.