(Reuters) - T-Mobile US Inc's TMUS.O quarterly profit jumped nearly sevenfold as the No. 3 U.S. wireless carrier recorded a $2.2 billion gain from recent changes in U.S. tax laws, but Wall Street analysts raised questions about the company's growth.

A T-Mobile logo is advertised on a building sign in Los Angeles, California, U.S., May 11, 2017. REUTERS/Mike Blake

T-Mobile has been using lower prices and added perks to take market share from larger rivals Verizon Communications Inc VZ.N and AT&T Inc T.N in a saturated market for wireless services. The company ended merger talks with rival Sprint Corp S.N late last year.

In 2018, T-Mobile said it expects to add between 2 million and 3 million subscribers who pay a monthly bill. Analysts called the numbers conservative.

“Management always guides conservatively,” said Jonathan Chaplin, an analyst at New Street Research, in a research note. He noted that in 2017, T-Mobile exceeded the high end of its initial expectations on subscriber additions.

But Craig Moffett, an analyst at MoffettNathanson, said in a note that cable companies' entry into the wireless market posed concerns about future growth. Comcast Corp CMCSA.O already sells mobile service and Charter Communications Inc CHTR.O plans to enter the market.

“It is not unreasonable to think that the two cable operators could capture a third of all market growth,” he wrote. “It is not unreasonable to assume that even go-go T-Mobile will see slower subscriber growth.”

Shares were down 2.5 percent to $60.50 in mid-morning trading.

The company said it added 891,000 phone subscribers in the fourth quarter, compared with 933,000 a year earlier.

T-Mobile’s net income was $2.71 billion, or $3.11 per share, in the quarter ended Dec. 31, up from $390 million, or 45 cents a share, a year earlier.

Excluding items, the company earned 48 cents per share.

Revenue rose 5.1 percent to $10.76 billion. Analysts, on average, were expecting revenue of $10.83 billion, according to Thomson Reuters I/B/E/S.

Last month, the wireless carrier said it had closed on its acquisition of Layer3 TV, a startup that has marketed itself as a next-generation cable service, for an undisclosed amount. It has plans to launch a new streaming service this year.