The union estimates that AT&T has cut 12,000 call-center jobs in the United States since 2011 while creating many such jobs overseas, but Mr. Master said the company had rebuffed a request for data that would clarify the extent of the practice and other changes to its work force.

Workers are also frustrated that they are being asked to pay more of their health-insurance costs, he added, and that changes in retail workers’ commissions have limited or reduced their take-home pay. As part of its next contract, the union wants to bar AT&T from changing its commission structure unilaterally.

Marty Richter, an AT&T spokesman, said the company was offering the wireless workers wage and pension increases and called the willingness to strike “baffling.” He said AT&T sales workers were well paid by industry standards, citing an average of over $68,000 in annual pay and benefits, roughly twice the figure for retail workers as a whole, according to PayScale, a company that tracks salary data.

Mr. Richter said that AT&T had successfully negotiated contracts covering nearly 130,000 workers since 2015, and that fewer than five of the company’s more than 25,000 retail workers lost jobs last year because of store closings. The union said that some company stores had been converted to noncompany stores and that much of the recent growth in the retail work force had occurred at noncompany stores.

With over 200,000 employees in the United States, AT&T is the country’s largest telecom company.

In addition to the call center and retail workers, the strikers include more than 15,000 employees on the company’s wireline side — which includes landlines and internet service — as well as about 2,000 employees of DirecTV, which the company acquired in 2015. The wireline and DirecTV workers have been without a new contract for over a year and held a one-day strike in March. The two groups’ contract negotiations are separate from those of the wireless workers.