Shares of these companies have soared over the past 12 months on expectations of deal talks, and are trading at up to 31 times forward earnings, versus the S&P 500 telecom services index’s 18 times.

The rich valuations could discourage acquirers, who also have to assume the risk that antitrust regulators may look askance at more consolidation in the sector after a wave of mergers in recent years, investment bankers and industry experts say.

“It seems as though valuations have already jumped to a near certainty a deal will be announced and approved. You have to ask yourself whether T-Mobile is going to be as eager to do a deal as Sprint,” said Craig Moffett, an analyst at MoffettNathanson.

Sprint (S) shares have risen 142% in the last 12 months, and T-Mobile (TMUS) shares have risen 65%. Both companies declined to comment on the possibility of a merger or how valuation considerations could be a factor.

Investors have long expected a deal between T-Mobile and Sprint, the third- and fourth-largest U.S. wireless service providers, anticipating cost cuts and other synergies in the range of $6 billion to $10 billion.

Reuters reported in February that Sprint’s controlling shareholder, SoftBank Group, was positioning itself for deal talks with T-Mobile’s top shareholder, Deutsche Telekom AG, once a U.S. government auction of airwaves spectrum ended.

Companies participating in the auction, which started last May, were banned from engaging in merger talks. The end of the auction last Thursday meant the FCC will lift the ban on April 27, when down payments are due from auction winners.

T-Mobile and satellite TV provider Dish (DISH) won the bulk of the spectrum, making them more attractive M&A targets, analysts said. T-Mobile now has more power to improve its network and support unlimited data packages for customers. Its financial results have also strengthened since it last held merger talks with Sprint in 2014.

DISH TO BUILD NETWORK

Controlled by Chairman and CEO Charlie Ergen, Dish faces an FCC deadline to use the spectrum by 2021 to build its first wireless network. Some investors say Ergen will likely want a partner to help share the cost of the investment, even though he has said the company can build the network by itself.

Analysts have viewed Dish as a likely target for Verizon Communications (VZ), since Dish would bring spectrum and its Internet TV business, Sling TV to the telecoms giant. Dish and Verizon declined to comment.

Verizon Chief Executive Lowell McAdam told investors in December that a deal with cable operator Charter Communications (CHTR) would make “industrial sense,” igniting takeover speculation.

With Charter, Verizon would gain a fiber and cable network across 49 million homes that could boost its wired network ahead of the advent of 5G wireless technology. Verizon and Charter declined to comment.

Charter’s controlling shareholder, billionaire John Malone’s Liberty Broadband, could be an obstacle to any deal. His lieutenant, Liberty Broadband Chief Executive Greg Maffei, said in March that “the hurdle around M&A is very high, because we are very enthused about our own plans.”

The price tag could also be an issue. Charter has a market capitalization of $101 billion and trades at 53 times forward earning estimates, far more expensive than Verizon’s 13 times.

“One of our principle concerns is that a deal would come with a high price target, and thus, be materially dilutive,” Barclays analyst Kannan Venkateshwar wrote in a research note.

Charter’s proxy statement to its shareholders shows that CEO Tom Rutledge has compensation incentives to take Charter’s share price to more than $564. The stock closed last week at $330.

FCC STANCE UNCLEAR

The stance of FCC Chairman Ajit Pai on these mergers is not clear. Pai is seen as a friend to major telecommunications companies, but price wars between Sprint and T-Mobile have helped to lower wireless prices for consumers so regulators may be reluctant to remove that competition.

It could be easier for regulators if cable and media group Comcast (CMCSA) wanted to buy a wireless company as that would preserve four major carriers in the market, analysts said.

Such a deal would be the most complementary for T-Mobile, according to Morningstar analyst Alex Zhao, since it would unite Comcast’s wired network with T-Mobile’s spectrum.

However, Comcast seemed to be forging ahead with a standalone wireless strategy, launching a mobile service and an unlimited data plan earlier this month using Verizon’s airwaves and buying $1.7 billion in the spectrum auction.

Comcast declined to comment.