The previous deadline had been Monday. The companies are working to win approval from the US Justice Department and Federal Communications Commission.

The No. 3 and No. 4 wireless providers announced their proposed merger on April 29, 2018.

Makan Delrahim, head of the US Justice Department's Antitrust Division, said in a television interview that meetings were ongoing as regulators review the deal.

"I have not made up my mind," he told CNBC.

"The investigation continues. We've requested some data from the companies that will be forthcoming. We don't have a set number of meetings or a time line."

"If the case is there for us to challenge a transaction or suggest changes, we will do that," he said.

The division is reviewing the argument that the deal would allow the combined company to produce a better, faster 5G, the next generation of wireless, he added.

"We're all one team, the antitrust division," he said.

"We will make a decision collectively. My job is to make sure that the analysis is done properly, make sure that the facts are there.