The contract for the 45,000 employees from Massachusetts to Washington, D.C., expired at midnight Saturday with the company and the workers unable to come to terms on issues including health care costs and pensions.
The dispute does not affect the company's wireless division. Verizon is the nation's largest wireless carrier.
In Philadelphia on Sunday morning, about a dozen red-shirted strikers and union officials were outside two downtown buildings.
Helga Weber, 45, of Abington, Pa., came prepared with the chair she bought for the workers' last strike in 2000, which lasted 18 days.
"We're just making a regular living, middle class - we're not making a fortune. We just want to keep what we have," said Weber, a 13-year employee.
Cliff Beckham, 55, of Upper Darby, said the walkout, his second in his more than 12 years with the company, would "definitely" be a hardship.
"I've got a mortgage to pay," he said. In addition to the $200 to $300 he said the union will provide, he hopes a part-time position with UPS will tide him over.
"Luckily, I have a night job," he said. "I'm glad I kept it."
Verizon employees who are members of the Communication Workers of America union picketed headquarters in New York City on Sunday morning, wearing red and holding signs with messages including "CWA on strike for middle-class jobs."
Vinnie Galvin, 56, said he and his fellow workers are the backbone of the industry. "Everybody needs to be wired and we're the people who do that," said the three-decades-plus veteran of the company.
"They're trying to bust us. ... This is stuff that it took us 40, 50 years to get."
The affected workers are responsible for maintaining and repairing traditional landlines, as well as installing FiOS, union spokesman Bob Master said.
Workers covered by the expired contract also include 10,000 represented by the International Brotherhood of Electrical Workers, who serve as telephone and repair technicians, customer service representatives, operators and more. Contract negotiations began June 22.
New York-based Verizon has 196,000 workers; 135,000 are non-union.
The company is asking for changes in the contract because it says its wireline business has been in decline for more than a decade as more people switch to using cellphones exclusively. It had 25 million landlines at the end of the second quarter, down from 26 million at the end of 2010. It has been selling off some of its landlines to other phone companies.
"It's not reflective of today's marketplace," Verizon spokesman Rich Young said of the contract. "Our union employees pay nothing toward their health care premiums. That's virtually unheard of."
Master said the company wanted worker concessions at a time when it was making billions in profits and top executives were making millions in salary.
"We have never seen such a sweeping attack on the quality of life of our members," he said. "This is an unprecedented and in our opinion completely unjustified attack on middle-class jobs."
Young refuted the union's contentions on Verizon's profits, saying that the wireline unit was declining overall despite the growth of products like FiOS.
Master said the strike could impact customers looking for installations or repairs to their service, but Young said Verizon had taken steps like training managers and retirees.
Young said Verizon would return to the table at any point, that it was up to the union. The union said workers would be prepared to return to work when Verizon demonstrated a willingness to bargain seriously.
Associated Press writers Samantha Gross in New York and Ron Todt in Philadelphia contributed to this report.