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(CNN) -- Millions of Verizon Wireless customers should have rebates on the way after the company said it incorrectly charged mobile phone users tens of millions of dollars for data they didn't use.

The U.S. Federal Communications Commission has confirmed it is investigating these "mystery fees," which could amount to $50 million in total.

A defect in Verizon's phone software caused at least 15 million wireless customers to be charged data fees, even if they didn't subscribe to data plans, Verizon said in a statement Sunday. Most of these customers received false charges of $2 to $6, which will be credited to customers' bills in October and November.

The rebate is said to be one of the largest paid by a wireless network.

<... p>"The majority of the data sessions involved minor data exchanges caused by software built into their phones; others included accessing certain Web links, which should not have incurred charges," Verizon's statement said. "We have addressed these issues to avoid unintended data charges in the future."

The FCC, meanwhile, has been investigating the issue since January when it first got word of the erroneous fees from consumers, according to a statement released Sunday by Michele Ellison, the FCC's enforcement bureau chief.

"We're gratified to see Verizon agree to finally repay its customers. But questions remain as to why it took Verizon two years to reimburse its customers and why greater disclosure and other corrective actions did not come much, much sooner," Ellison said.

"The Enforcement Bureau will continue to explore these issues, including the possibility of additional penalties, to ensure that all companies prioritize the interests of consumers when billing problems occur."

Verizon, one of the largest wireless network providers in the U.S., said the fees were charged to customers "over the past several years." Current customers will receive credits on their upcoming bills, and former customers will get checks in the mail, the wireless company said.

Some of the fees were charged when customers without data plans opened mobile Web links, even though they were not receiving data and should not have incurred charges, the company said. Other fees were levied without any actions from mobile phone users, according to Verizon's statement.

"Customers who did not have data plans were billed for data sessions on their phones that they did not initiate," the company said. "These customers would normally have been billed at the standard rate of $1.99 per megabyte for any data they chose to access from their phones."

Reports of incorrect Verizon data fees were initially surfaced in 2009 in The New York Times and The Cleveland Plain Dealer.

Based on Verizon's report, the overall size of the fee rebate could range from about $30 million to $90 million. The company said some fees returned to mobile phone users may exceed the average range of $2 to $6 per customer.

Tom Pica, a Verizon spokesman, declined to comment on the total size of the rebate.

The FCC said it will continue its investigation.

"Our role is to protect American consumers and give them a voice," the commission said. "Consumers have a right to receive straight bills and to get straight answers when they question them."