The revival of T-Mobile US has given cellular customers more viable options, but that doesn't mean the company is going to become a sustainable business.

The carrier has lost money in five of the past six quarters and owner Deutsche Telekom is not bullish on its prospects unless it can merge with another company, according to an interview in Re/code today.

Deutsche Telekom CEO Timotheus Höttges said that “We have done what we had to do," Re/code reported. “We had built an infrastructure and this infrastructure had to get utilized and we did that with very aggressive promotions.”

But the company needs to invest $4 billion to $5 billion each year to keep pace with AT&T and Verizon Wireless, which are likely to continue boosting their network capacity through spectrum auctions (although Federal Communications Commission Chairman Tom Wheeler wants to make sure smaller carriers have a chance too). Höttges "admitted that T-Mobile’s current approach is not sustainable" in the long run, Re/code reported.

“The question is always the economics in the long term... and earning appropriate money,” Höttges said. “You have to earn your money back at one point in time.”

Deutsche Telekom tried to sell T-Mobile to Sprint last year but Sprint pulled out of the deal when it became clear that US regulatory authorities wanted to preserve four nationwide carriers. A French company also tried to buy T-Mobile but abandoned the plan because Deutsche Telekom wasn't interested.

“I was intrigued by the idea of having a combination with Sprint and being the ‘super-maverick’ in the market,” Höttges told Re/code. “I hope that the political environment will change at one point in time.”

T-Mobile CEO John Legere has been outspoken in criticizing his larger competitors, but Höttges said that Legere's "management style will never be adaptable to Germany," where Deutsche Telekom is based.