Sources told the newspaper Sprint is studying industry regulations and could make a move for T-Mobile in the first half of 2014 in a deal that could be worth more than $20 billion.
Verizon Wireless and AT&T Inc. are the nation's top two carriers, with Sprint and T-Mobile at No. 3 and No. 4, respectively.
Antitrust authorities likely would oppose the merger, believing the lack of a fourth national competitor could force consumers to pay more, the newspaper said.
The Justice Department in 2011 nixed AT&T's attempt to acquire T-Mobile.
SoftBank Chief Executive Officer Masayoshi Son bought control of Sprint this year and is said to be interested in expanding in the U.S. market.