July 14 (Bloomberg) -- Centennial Communications Corp., the mobile-phone company being taken over by AT&T Inc., fell the most in eight months after Stifel Nicolaus & Co. said the carriers may have to review the terms of the deal.

Centennial, based in Wall, New Jersey, fell 78 cents, or 9.6 percent, to $7.35 at 4 p.m. New York time in Nasdaq Stock Market trading, the biggest one-day drop since Nov. 5. Dallas- based AT&T agreed to buy Centennial for $944 million, or $8.50 a share, in November.

The U.S. Justice Department and the Federal Communications Commission are scrutinizing the transaction. The combined company would control 40 percent of Puerto Rico’s wireless market, which may prompt regulators to force AT&T to divest some assets, Stifel analyst Christopher King said today in an interview. Selling the Puerto Rican wireless business, which accounts for about a third of Centennial’s revenue, may cause AT&T to cut its offer, he said.

“We’re clearly in a different antitrust environment,” said King, who is based in Baltimore. “This is certainly the first significant opportunity that the Obama administration will have had in the telecom sector to lay down the law.”

AT&T hasn’t been asked to divest assets in Puerto Rico, said McCall Butler, a spokeswoman. Steve Kunszabo, director of investor relations for Centennial, didn’t immediately return a call seeking comment.

AT&T, the second-biggest U.S. wireless carrier, and Verizon Communications Inc., its larger rival, swapped assets in May to appease concerns that AT&T’s Centennial purchase would hurt competition.

King advises investors to buy AT&T shares and doesn’t own them. The stock fell 32 cents to $23.45 today in New York Stock Exchange composite trading.

To contact the reporter on this story: Julie Alnwick in New York at jalnwick@bloomberg.net; Amy Thomson in New York at athomson6@bloomberg.net

Last Updated: July 14, 2009 18:06 EDT