(Reuters) - Sprint Corp S.N has restarted talks to merge with T-Mobile US Inc TMUS.O, people familiar with the matter said on Tuesday, the latest effort to bring together the fourth and third largest U.S. wireless carriers.

FILE PHOTO: Smartphones with the logos of T-Mobile and Sprint are seen in this illustration taken September 19, 2017. REUTERS/Dado Ruvic/Illustration

The combined company would have more than 127 million customers and could create more formidable competition for the No.1 and No.2 wireless players, Verizon Communications Inc VZ.N and AT&T Inc T.N, amid a race to expand offerings in 5G, the next generation of wireless technology.

A previous round of talks ended in November over valuation disagreements but Tim Hoettges, CEO of T-Mobile's German parent Deutsche Telekom DTEGn.DE, left the door open at the time, saying: "You always meet twice in life."

Since then, Sprint’s shares have lost more than a fifth of their value amid questions about how the company can compete effectively under the weight of its long-term debt of more than $32 billion.

Sprint's majority owner, SoftBank Group Corp 9984.T, has been looking to trim its debt, which reached 15.8 trillion yen ($147 billion) as of the end of December. It has said it is planning to raise cash by taking its Japanese mobile phone unit public this year.

Sprint and T-Mobile decided to restart talks partly because they want to share the burden of investing in their networks, the sources said. The negotiations are at an early stage, they added.

A key consideration in the talks is Deutsche Telekom’s ability to consolidate T-Mobile’s earnings, one of the sources said. Deutsche Telekom owns 63 percent of T-Mobile, which has emerged as one of its more prized assets. Deutsche Telekom would likely have to put in new money toward a merger for its stake to remain above 50 percent.

The sources asked not to be identified because the matter is confidential. Sprint and Deutsche Telekom declined to comment, while T-Mobile did not respond to requests for comment.

“I don’t see how Deutsche Telekom agrees to a transaction in which it relinquishes control,” said Dhananjay Mirchandani, a telecoms analyst at Bernstein.

Mirchandani said any control premium would probably be palatable to Deutsche Telekom’s shareholders. He doubted, however, that the case for a deal had become more compelling since November.

Shares of Sprint closed up more than 17 percent at $6.02, giving the company a market capitalization of $25 billion, after the Wall Street Journal first reported on the new talks.

T-Mobile rose 5.67 percent to $63.13, valuing it at $54 billion.

In Tokyo, Softbank gained 3.5 percent to 7,980 yen, while Deutsche Telekom rallied by 4 percent in early European trading to 14.11 euros, a three-month high.

SEEKING OPTIONS

Failure to clinch a deal last November left SoftBank Chief Executive Officer Masayoshi Son, a dealmaker who raised close to $100 billion for his Vision Fund to invest in technology companies, in search of other options for Sprint.

“SoftBank has to change the way they approach this,” Recon Analytics analyst Roger Entner said. “Unless they have changed their minds, (and admitted) that they are indeed the junior partner, nothing has changed.”

Even though Sprint’s customer base has expanded under CEO Marcelo Claure, growth has been driven by discounting. Analysts say that, without T-Mobile, Sprint lacks the scale needed to invest in its network and to compete in a saturated market.

“It is impossible for Sprint to sustain on its own, and the same problems still exists with SoftBank and Sprint not comfortable with a minority stake,” MoffettNathanson LLC analyst Craig Moffett said. “But ultimately you have to believe that these two companies will end up together even if the path to get there is torturous.”

T-Mobile has fared better than Sprint, even if it remains a distant third to Verizon and AT&T. It has managed to score sustained market share gains, as innovative offerings, improving network performance and good customer service attract new customers, according to Moody’s Investors Service Inc.

T-Mobile became the first major U.S. carrier to eliminate two-year contracts, a shift quickly embraced by consumers and copied by competitors. The company has also badgered rivals with its unlimited data plans.

REGULATORY CONCERNS

Another roadblock to the deal could be regulatory hurdles. Sprint’s and T-Mobile’s first round of merger talks ended in 2014 after the Obama administration expressed antitrust concerns about the deal.

It was not immediately clear how the Trump administration would view the combination. AT&T agreed to acquire U.S. media company Time Warner Inc TWX.N in October 2016 for $85 billion. The U.S. Department of Justice has sued to block the deal over concerns about the companies' pricing power in the media market. AT&T and Time Warner are currently defending their deal in court.

Caroline Holland, a veteran of the Justice Department’s Antitrust Division who is now a Mozilla tech policy fellow, said that Sprint and T-Mobile would likely argue that their combination would mean more effective competition for Verizon and AT&T.

“They are probably betting that they can make a competent and convincing argument that it’s more important to have a stronger No. 3. They’ll make a case that Sprint hasn’t been a strong competitor,” she said.

“I have a very healthy skepticism. I think four competitors has benefited consumers.”

Analysts have said a combined company would cut costs by reducing cell-tower sites; shares of operators American Tower AMT.N, Crown Castle International CCI.N and SBA Communications SBAC.O fell on the news.