AT&T Corp unveiled a discount plan in Ameritech Corp’s Illinois market that may set the stage for national price wars, the Wall Street Journal said. In what is being seen as a pre- emptive strike at phone companies aiming to compete with AT&T in long distance, AT&T will offer new customers three months of free, unlimited local toll calling in the region. It said it would extend discounts on its local rates thereafter and that customers could apply their local-toll calls to their current AT&T discount plans. AT&T is also considering new price cuts in Connecticut as a way to thwart the success of Southern New England Telecommunications Corp