Unions representing 45,000 Verizon Communications (VZ.N) workers called for a strike as negotiations failed for a new labor contract that expired at midnight on Saturday.
Verizon and the unions -- The Communications Workers of America and the International Brotherhood of Electrical Workers - had been in talks since late June.
The workers who went on strike on Sunday are technicians and customer support employees in Verizon's wireline business, which provides Internet and land phone lines to homes and businesses in the U.S. Northeast.
Verizon is looking to keep costs in check at its wireline business, which has been declining for a decade as customers have disconnected their home phones in favor of cellphone and Internet services.
The two sides were unable to agree on issues related to healthcare contributions, pension plans and work rules, according to Verizon and the CWA.
Verizon said it had trained tens of thousands of employees, from retirees to management, to fill the role of the workers who are now on strike.
"We are confident that we have the talent and resources in place to meet the needs and demands of our customers," Marc C. Reed, Verizon's executive vice-president of human resources, said in a statement.
The CWA says the contributions to healthcare that Verizon wants the union members to make were unacceptable, and that increases in deductibles would make the proposed healthcare plan unaffordable.
The unions had warned of a strike and began to mobilize support among their members in early July. Verizon did not budge from the set of changes it asked for, which the unions were demanding the company take off the table.
With only hours left under their contract late on Saturday night, both unions issued statements accusing Verizon of not "getting serious" in negotiations. The CWA, which represents 35,000 workers under contract, said all the major concessions Verizon was asking for were yet to be agreed upon.
But the CWA said the profitable company, which is one of the two big U.S. telephone network operators, is asking for far too many concessions from affected workers, who include technical and customer service employees in Verizon's wireline business.
Among the changes it is seeking, Verizon said it wants to freeze employee pension plans and replace them with an "enhanced 401(k) plan." It also wants workers to contribute to healthcare insurance premiums.
CWA members in July authorized union leaders to call a strike if negotiations stalled, with 91 percent of votes in favor of the strike.
Verizon has 93,000 workers in its wireline business, of whom 58,000 are unionized. Including its Verizon Wireless venture with Vodafone Group Plc (VOD.L), the company's total workforce is 196,000 employees.