Telecommunications giant AT&T has broken federal law by unilaterally forcing technicians to do work outside their areas of expertise, according to the union representing 17,000 workers in California and Nevada who walked off the job Wednesday.

The union is also taking aim at AT&T’s outsourcing and offshoring of jobs.

Joining technicians in the strike are call center workers. The striking workers are employed in landline services, not wireless.

AT&T criticized the job action.

“A walkout is not in anybody’s best interest, and it’s unfortunate that the union chose to do that,” said company spokesman Steven Maviglio.

Picket lines, according to the Communications Workers of America, which represents the workers, have gone up outside AT&T offices and call centers across California and Nevada, with larger actions taking place at the firm’s offices in San Francisco, Los Angeles and San Diego.

The striking employees have been without a contract for nearly a year, according to the union.

“Workers are growing increasingly frustrated with the company’s attempts to short-change California and Nevada working families and communities,” a union news release said.

Nationally, more than 21,000 AT&T wireless-services workers are working on a contract extension that can be ended with three days’ notice.

“In the last week, wireless workers have intensified their calls on AT&T executives to end offshoring and outsourcing and have joined rallies and pickets coast to coast demanding good jobs that support their families and quality customer service,” the union said.

AT&T highlighted the compensation received by some in the striking workforce.

“The average AT&T core network technician in this contract makes nearly $150,000 in wages and benefits,” Maviglio said. “We are not proposing to reduce the wages of any employees in these contracts, and we remain committed to providing great benefits.

“We’re the country’s largest employer of full-time union labor, and our goal in these negotiations is to continue to provide our employees with high-quality union careers with wages and benefits that are among the best in the country.”

The firm has reached 28 labor agreements since 2015, covering close to 123,000 employees, Maviglio added.

AT&T brought in $42 billion in revenue in the fourth quarter of last year, the union noted.

“Despite the financial success, the company is asking its workers to do more for less — keeping them from their families with unpredictable overtime, undercutting pay and advancement, offshoring good jobs, and pushing more health care costs onto employees,” the union said.

Last week, dozens of mayors and other elected officials from the Bay Area and around California issued a letter claiming AT&T was denying high-speed broadband internet and reliable landline service to communities while reaping huge profits and firing workers.

AT&T said early Wednesday afternoon that it was in discussions with the union “to get these employees back to work as soon as possible.” In the meantime, the firm was “very prepared to continue serving customers,” spokesman Maviglio said.

“We’re a customer service company, and we plan for all contingencies, whether related to weather, natural disasters, work stoppages or any other factors.”