On Wednesday, 40,000 Verizon workers across six US states went on strike after contract negotiations failed to produce a settlement.
The unions organising the action represent customer service and network technicians.
Verizon called the strike “regrettable” and said it had brought in 10,000 non-union workers.
Presidential candidate Bernie Sanders spoke at a protest in New York in praise of the workers.
He 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.
CWA representative Bob Master said the new workers would not be able to make up the workload left by those out on strike.
“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],” he 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.
“It’s regrettable that union leaders have called a strike, a move that hurts all of our employees,” Marc Reed, Verizon’s chief administrative officer, said in a statement.
During the last contract talks in 2011 workers also went out on strike. A deal was eventually reached through mediation.
Verizon said it has asked the Federal Mediation and Conciliation Service to step in this time around. Mr Master called that move a “diversionary tactic”.
The strike could effect Verizon’s television, phone and Fios Internet businesses, is not expected to impact Verizon wireless, which made up 29% of the company’s 2015 revenue.