With AT&T agreeing to buy T-Mobile USA for $39 billion, the future of Sprint Nextel just got fuzzier.

Over the past several months, Sprint had held its own talks with Deutsche Telekom, the owner of T-Mobile, over a potential deal, as part of an effort to gain sorely needed scale and wireless spectrum, according to people with knowledge of the talks, which were intended to be private. But Deutsche Telekom’s discussions with AT&T moved more quickly this month, and the two reached a preliminary agreement last week, one of these people said.

If the deal is approved, AT&T would again leapfrog Verizon Wireless to become the country’s biggest cellphone service provider, and Sprint’s strategic options would become more limited.

Sprint Nextel, the product of an ill-fated merger in 2005, risks being further eclipsed by Verizon and the new AT&T, which together would boast 230.3 million customers in the United States. By contrast, Sprint, based in Overland Park, Kan., has just under 50 million customers.

“That means three out of every four wireless subscriptions belongs to those two companies,” said Charles S. Golvin, an analyst at Forrester Research. “That is going to be very challenging for Sprint.”

Under its current chief executive, Dan Hesse, Sprint has managed to lower its customer defection rate, in large part through improving its customer service and its product lineup. But the company has struggled to translate that into a healthier financial picture: Its annual loss widened to about $3.5 billion last year, as revenue declined slightly.

The idea behind combining Sprint and T-Mobile was to add much-needed scale. Both have competed against their larger rivals on price, cutting into revenue.

Underpinning the interest for T-Mobile is concern by the major service providers about limited wireless spectrum. As more and more Americans adopt devices like the iPhone, Android smartphones and tablets like the iPad, cellphone carriers are urgently seeking to add more and more capacity to handle customers’ data use.

But the talks were weighed down by several major issues, including Sprint’s and Deutsche Telekom’s differing valuations of T-Mobile. Another, more technical issue revolved around the two companies’ use of different cellphone technologies. Sprint currently uses a standard known as CDMA, which is limited mainly to the United States and a handful of other countries. T-Mobile relied on the more common GSM technology.

In the race to set up the next generation of wireless networks, Sprint is also increasingly alone in its reliance on the WiMax standard. AT&T and Verizon have both chosen an alternative called LTE. For its part, T-Mobile lacked the spectrum necessary to develop its own next-generation network, a major driver behind its search for a merger partner.

With AT&T now adding T-Mobile’s 33.7 million subscribers to its fold, hardware makers could lose interest in creating products for companies other than the biggest players. Sprint may find it difficult to add cutting-edge new handsets.

A larger AT&T could also strike better prices for the handsets it sells, which would allow it to expand its profit margins or reduce the prices on its products.

Sprint is likely to be forced to compete more aggressively on price to differentiate itself and attract new subscribers.

“This puts them into a much weaker No. 3 position,” Mr. Golvin said.