About 17,000 AT&T workers walked off the job Wednesday.
The strike began amid ongoing negotiations between AT&T and landline workers in California and Nevada who are represented by the Communications Workers of America union.
The union said via Facebook that employees are angry that AT&T "recently made a unilateral change in job requirements [for landline technicians] without the agreement of the union." The two parties failed to reach an agreement after several days of talks.
AT&T said Wednesday that the strike does not include workers in wireless departments, and that the company is engaged in conversations with CWA in an attempt to get "employees back to work as soon as possible."
"A walkout is not in anybody's best interest," AT&T spokesperson Marty Richter said, but the company is "very prepared to continue serving customers" despite the strike.
AT&T workers in California and Nevada have been working under an expired contract for almost a year. CWA said last week that the company has refused to "bargain fairly."
The union is also angry that AT&T has cut thousands of landline technician and call center jobs in the region in recent years. Elected officials in both states took up the issue by sending a letter to AT&T CEO Randall Stephensen, the CWA said last week.
The letter says the company is "harming communities by cutting thousands of good, middle-class jobs." It also accuses AT&T of dragging its feet on promises to provide high-speed internet throughout rural communities in California and Nevada.
Earlier this month, AT&T reached a tentative separate deal with unionized workers in the southwest that included a promise to bring home 3,000 jobs that have been outsourced to workers overseas.