A lawsuit the ATM operators filed Wednesday claims MasterCard and Visa impose contract terms that prevent them from charging less when consumers use cards that can tap networks other than MasterCard’s and Visa’s.

Independent ATM operators’ machines are not affiliated with banks and are most typically found at drug stores, convenience stores and gas stations.

The ATM operators are seeking class-action status for their antitrust claims, which lift the curtain on the complex, behind-the-scenes dealings in how debit card transactions are processed.

Representatives from MasterCard Inc. and Visa Inc. declined to comment.

Although many consumers identify the plastic in their wallets as either MasterCard or Visa, it is actually banks and credit unions that issue those cards. MasterCard and Visa simply provide the networks that process transactions; their role is analogous to the operators of phone lines or wireless networks.

There are smaller payment processors too for debit card transactions, such as Star, Nyce and Pulse, which is owned by Discover. But MasterCard and Visa are the predominant players in the arena.

Even if a card bears a MasterCard or Visa logo, banks can choose more than one payment network on which their cards can function. ATM operators select which networks they tap to complete a transaction.

The lawsuit claims MasterCard and Visa force ATM operators to charge consumers the same fee all the time, even when their cards can access smaller networks that are cheaper for the ATM operators to use.

If ATM operators could charge those customers lower fees, consumers would start asking their banks for cards equipped to use the cheaper networks, said Jonathan Rubin of Rubin PLLC, the Washington, D.C. antitrust law firm that filed the lawsuit.

“It protects them from competition,” he said. “Some people are not going to use an ATM because of the $2.50 fee. By discouraging those customers, (MasterCard and Visa) are holding down the volume of ATM transactions.”

Rubin said his clients operate about half the 400,000 ATMs in the country.

The plaintiffs in the case are the trade group the National ATM Council and several independent ATM operators. The lawsuit, filed in U.S. District Court in Washington, D.C., seeks “tens of millions of dollars” in damages and compensation for violations of antitrust laws.