More than 40,000 Verizon workers across six US states went on strike on April 13 after contract negotiations failed to produce a settlement.
The unions organizing the action represent customer service and network technicians.
Verizon called the strike “regrettable” and said it had brought in 10,000 non-union workers.
Democratic presidential hopeful Bernie Sanders spoke at a protest in New York in praise of the workers.
Bernie Sanders said they were “standing up to this powerful special interest”.
The workers are members of the Communications Workers of America and the International Brotherhood of Electrical Workers.
“There’s no way that these 10,000 people … can make up for 40,000 people who have decades of experience [in highly technical jobs],” Bob Master said.
The union workers have been without a contract since August and negotiations have stuck on Verizon’s plan to cut healthcare and pension benefits over a three-year period.
In a statement, Marc Reed, Verizon’s chief administrative officer, said: “It’s regrettable that union leaders have called a strike, a move that hurts all of our employees.”
During the last contract talks in 2011 workers also went out on strike. A deal was eventually reached through mediation.
The strike could effect Verizon’s television, phone and Fios Internet businesses.
It is not expected to impact Verizon wireless, which made up 29% of the company’s 2015 revenue.