The International Brotherhood of Electrical Workers Local 3 said the strike was caused by wage, pension and health care coverage disputes with Charter.
At stake is a battle over the quality of union jobs at AT&T.
Another major point of contention for the workers is the outsourcing of labor. Others are employed in call centers spread out across the country.
The two groups' contract negotiations are separate from those of the wireless workers.
"We've been bargaining with AT&T for over a year", CWA president Chris Shelton told the rally in Silicon Valley. CWA represents about 150,000 of them.
In California and Nevada, around 17,000 AT&T workers who provide phone, landline and cable services have been working without a contract for more than a year.
No one from the company could be reached for comment.
The company and the union have held contract negotiations for almost four months of bargaining, according to the statement, which said workers are fighting for wage increases to cover rising healthcare costs, basic job security, affordable healthcare, and a fair scheduling policy.
Among the concessions being sought by the CWA are protections for employee wages and benefits. The contracts at stake account for only 13% of the company's employees, he added.
"They're trying to close up corporate stores to open up authorized retailer stores".
AT&T is the single largest telecom corporation in the US, listing $164 billon in sales and 135 million wireless customers. "We are what help make AT&T the company that it is and, I think, that will get our message across", said Travis Daniel, picketer.
Employees at several area AT&T stores, including the one at the Eastwood Mall in Niles, and the call center in Boardman said they plan to join the picket line.
AT&T, for its part, said that the company is prepared for workers to walk off the job but continues to negotiate.