For an M&A deal that could yield big synergies but also faces big regulatory risks, any decision on whether or not to push ahead with merger talks comes down to a risk/reward calculation. Are the benefits of a deal strong enough, and the odds of it being approved high enough, to offset all the costs -- termination fees, distracted management, missed opportunities for other deals -- associated with a potential rejection?

Under the Trump Administration, T-Mobile US Inc. (TMUS) - Get Report  and Sprint Corp. (S) - Get Report  apparently feel the answer to that question, as it relates to a tie-up between the #3 and #4 U.S. mobile carriers, is "yes." But T-Mobile's risk/reward calculations are probably a lot different than Sprint's, given its much healthier financial and competitive position, and just because the risk of a rejection has dropped doesn't mean it's no longer significant.

On Tuesday morning, September 19, CNBC and Reuters both reported that T-Mobile and Sprint have resumed talks and were discussing an all-stock deal that (unsurprisingly) would give T-Mobile a majority stake in the combined company. Sources cautioned that any deal is still weeks away and isn't guaranteed to happen. CNBC added an exchange ratio for shares hasn't been agreed to yet.

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