SoftBank is looking to have its recently acquired US unit Sprint take a majority stake in T-Mobile from the latter's parent Deutsche Telekom in the financial year starting April, one of the sources added.
The deal, which media reports have said would be valued at about $20 billion in line with the $21.6 billion SoftBank paid for Sprint this summer would help the Tokyo-based company leapfrog US rivals Verizon and AT&T to become the world's No. 2 mobile carrier by revenue.
It would also bring SoftBank CEO Masayoshi Son closer to his ambition of building the world's biggest mobile internet company if he can overcome US regulators' expected concerns about competition issues.
More than the financial and funding aspects, there are likely concerns in the United States about how much Son, head of a foreign company, can really open up mobile infrastructure there, and whether the deal would obstruct healthy competition," a banking source in Tokyo said.
Sprint has been interested in combining with T-Mobile for years and top executives from both companies have said that consolidation was necessary in the US wireless market, as cooperation in infrastructure and equipment orders would create a stronger rival against the two biggest players.
Both the US Federal Communications Commission, which turned down AT&T's application to acquire T-Mobile in 2011 due to competition concerns, and the Justice Department chiefs have signalled they will take a hard line in scrutinising consolidation bids.
Son has met with at least five banks to discuss financing, Bloomberg reported over the weekend, including Credit Suisse Group, Mizuho Bank, Goldman Sachs Group and Deutsche Bank.
Steering SoftBank through the acquisition would be a test, added one banking source, for the finance team following the death in October of Kazuhiko Kasai, a former banker who was seen as Son's right-hand man.
SoftBank shares dropped 0.5% to 8,770 yen on Wednesday, while T-Mobile shares closed up 1% at $32.19 on Tuesday.
SoftBank's shares have rallied in recent weeks to their highest since the dot-com bubble burst more than a decade ago, as investors buy into Son's aggressive acquisition and growth strategy. They include hedge fund manager Daniel Loeb, who disclosed last month that he had invested more than $1 billion in the company.