The government wanted a delay to stop planes falling out of the sky if the new 5G service was bought in around airports.
However, AT&T and Verizon said that did not matter as they would take “enhanced measures to avoid warned disruptions of air travel."
US Transportation Secretary Pete Buttigieg and FAA Administrator Steve Dickson had asked the companies on New Year's Eve to put off Wednesday's launch of the new wireless service for two additional weeks while their agencies address concerns that the new signals could interfere with some types of aircraft equipment.
Unless those issues are resolved, air travellers will face "widespread and unacceptable disruption as airplanes divert to other cities or flights are cancelled, causing ripple effects throughout the US air transportation," Buttigieg and Dickson wrote in their letter.
The wireless carriers' rebuff is the latest step in weeks of an escalating standoff between the aviation and telecommunication sectors — a year after the mobile phone companies spent more than $80 billion to buy licenses for the 5G-friendly C-band airwaves at a Federal Communications Commission auction.
The fight over the Trump-era initiative has drawn in multiple agencies and the White House, with airlines pushing for an emergency stay even after the FCC insisted that the wireless companies can safely use the airwaves.
The FAA on Sunday said it's reviewing the carriers' letter but added, "U.S. aviation safety standards will guide our next actions."
Verizon and AT&T said that while they would bring in more extensive mitigation measures through 5 July they outright rejected the idea of postponing the launch of the new 5G service.
"Agreeing to your proposal would not only be an unprecedented and unwarranted circumvention of the due process and checks and balances carefully crafted in the structure of our democracy, but an irresponsible abdication of the operating control required to deploy world-class and globally competitive communications networks that are every bit as essential to our country's economic vitality, public safety and national interests as the airline industry," the two companies' CEOs wrote.