SoftBank (SFTBY) said its nine-month profit doubled from the same period last year as the Japanese wireless carrier enjoyed boosts from its domestic broadband business and a turnaround at is U.S. Sprint (S) - Get Reportdivision.

The Tokyo-based telecom-to-semiconductor group said net profit soared 99.9% to ¥857 billion ($7.6 billion) despite a revenue drop of 0.3% to ¥6.6 trillion in the nine months through December. In the third quarter alone, net profit came in at ¥91.2 billion on revenue of ¥2.3 trillion. Operating profit jumped 18% in the nine months to ¥950 billion, the company said.

SoftBank benefited from growth in both its domestic and overseas businesses, with revenue from its domestic Japanese telecom business advancing 2% as a 57% revenue jump in its broadband business comfortably offset declines suffered by the mobile and fixed telecom businesses.

SoftBank noted Sprint that while the Overland Park, Kansas-based Sprint has remained more-or-less in the red since Softbank's acquisition in 2013, its cost cutting is starting to pay off.

Last week, Sprint said its net loss for the three months ending in December narrowed to $479 million from $836 million a year earlier, while earnings at the operating level moved into the black to $311 million from a loss of $197 million.

Revenue for the quarter rose to $8.5 billion from $8.1 billion, topping Wall Street estimates, and CEO Marcelo Claure touted the 368,000 net additions of post-paid phones, which have traditional monthly phone bills, during his conference call with analysts. Sprint topped Verizon (VZ) - Get Report and AT&T (T) - Get Report for the fourth straight quarter and notched its highest subscriber gains in four years during what Claure called the most competitive quarter he has seen at the helm of the group.

SoftBank's debt, however, remains a concern, and was pegged at ¥14.9 trillion at the end of December, up from ¥14.3 trillion at the end of September and ¥11.9 trillion at the end of March 2016. The group is rated Ba1 by Moody's Investors Service and BB+ by Standard & Poor's.

SoftBank shares closed at ¥8,684 in Tokyo, up 0.23% on the session and have advanced nearly 35% over the past three months.