Once the 72-hour notice period expires, the workers could go out on strike, the Communications Workers of America Union said. The 21,000 workers spread across 36 states and Washington, D.C., have been working without a contract since February, unable to resolve their concerns about wages, health care costs, and job security against outsourcing.

“We have given AT&T every opportunity to show their commitment to finding common ground and a fair contract and, quite frankly, have been very patient about it,” Dennis Trainor, vice president of CWA’s District 1, said in a statement. “The pieces are all in place now, and we’ll strike if necessary.”

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Negotiations are continuing and AT&T is “confident a fair agreement can be reached,” a spokesman said. The contract covers U.S. jobs averaging nearly $70,000 in pay and benefits, he said. “Our labor agreements include annual wage increases and we’re confident employees will be better off in their new contract,” the spokesman said.

With tensions rising with the wireless workers, AT&T also faces another tough ongoing negotiation covering about 17,000 workers in AT&T’s wired telephone, cable, and Internet business in California and Nevada. The workers went out on a one-day grievance strike last month and have been holding large protest rallies.

The two contentious fights stand in stark contrast to the carrier’s long run of labor peace. AT&T hasn’t suffered a labor strike since 2012, and that lasted only two days. And a recent four-year contract ratified this month by 20,000 workers in Arkansas, Kansas, Missouri, Oklahoma, and Texas marked the 28th straight deal approved by AT&T and its unionized workers since the start of 2015. As part of the recent southern workers deal, AT&T agreed to hire 3,000 people locally for jobs that had been outsourced, mostly overseas.

But labor relations have been getting increasingly tense in the telecommunications industry since last year’s strike by Verizon workers resulted in higher pay and better job security than the company was initially offering.

AT&T’s (T) wireless workers have said they are concerned that AT&T’s offers seek to make them pay more for health care, eliminate pension benefits for new hires, and reduce their number of sick days. Like the Verizon (VZ) workers who went out on strike, AT&T’s workers also highlighted their employer’s outsourcing of call center jobs outside of the country.

AT&T, which is under pressure from Wall Street to cut costs amid slowing growth in its wired and wireless telephone operations, has said it is not seeking to cut employee pay or take away benefits.

(Update: This story was updated on April 28 to correct that the wireless workers contract expired in February.)