U.S. Justice Department officials told Sprint CEO Dan Hesse and SoftBank CEO Masayoshi Son that any potential Sprint/T-Mobile merger would be met with “skepticism.”

The Wall Street Journal spoke with individuals briefed on the conversation who said the meeting reinforced Son’s seriousness in making the merger happen.

Japanese carrier SoftBank last year bought a majority stake in Sprint and now the combined company is said to have its eye on Deutsche Telekom’s (DT) 67-percent stake in T-Mobile. Bloomberg last week said that SoftBank and DT have entered direct talks regarding the merger, with issues like the value of DT’s share and the amount of a potential break-up fee attached to the deal presenting immediate hurdles. Also up for discussion is just how Sprint and T-Mobile could potentially be integrated.

Sprint has reportedly received assurances that lenders are prepared to help finance the deal, valued at around $50 billion. That total includes $31 billion for T-Mobile and $20 in T-Mobile debt the combined company would assume.

It almost goes without saying that U.S. regulators, who place high value on a four-competitor U.S. wireless market, would be very skeptical of a deal that would disrupt that landscape.

With T-Mobile’s recent successes and the buzz it’s built around un-carrier branding pricing and plan moves, it would seem the carrier would be content to go it alone. But CEO John Legere showed his interest in merging with Sprint, saying the combination could pose a real threat to the “duopoly” Verizon and AT&T currently enjoy in the U.S. wireless market.