The Wall Street Journal originally reported that the talks were underway.
The two sides have agreed that after the merger, T-Mobile Chief Executive John Legere would serve as the CEO and Dish CEO Charlie Ergen would become the combined company's chairman.
However, a purchase price and the mix of cash and stock that would be used to pay for a deal remain unresolved.
T-Mobile has a market capitalization of about $31.06 billion, while Dish has a market capitalization of $32.77 billion.
Reuters could not immediately reach Dish or T-Mobile for comment outside regular U.S. business hours.
Legere said in February that it made sense for T-Mobile to team up with Dish, the second-largest satellite TV company in the United States.
"Dish and we, that makes some sense," Legere told investors at Deutsche Telekom's capital markets day. Deutsche Telekom owns about 66 percent of T-Mobile US.
Deutsche Telekom last year tried to sell T-Mobile US to Sprint but the No. 3 U.S. carrier dropped its bid after regulatory resistance. French low-cost telecoms operator Iliad also abandoned its attempt to buy T-Mobile last October.
T-Mobile rival AT&T is close to wrapping up its $49 billion deal for Dish rival DirecTV, while Charter Communications is seeking to remake the U.S. cable television industry by acquiring larger rival Time Warner Cable for $56 billion.
—CNBC's David Faber contributed to this report.