BY ANTON TROIANOVSKI, GREG BENSINGER AND AMY SCHATZ

AT&T Inc. signaled for the first time that its planned $39 billion acquisition of T-Mobile USA is more likely to fail than to succeed, saying Thursday it would set aside $4 billion in this year's final quarter to cover the potential cost of the deal falling apart.

The move came after Federal Communications Commission Chairman Julius Genachowski said this week he would seek a rare, trial-like hearing on the merger, which would add months of arguments and another big hurdle for the controversial deal.

AT&T and T-Mobile parent Deutsche Telekom AG responded Thursday morning by pulling their application for merger ...