The whispers among employees had been around for years. They finally heard some facts during a conference call in June led by managers in Wells Fargo & Co.’s foreign-exchange operation: Some of its business customers had been cheated, according to two employees who were on the call.

An internal review showed that out of roughly 300 fee agreements based on anything from informal handshakes to emails to signed documents, only about 35 companies were charged the actual price they had been offered for currency trades handled by Wells Fargo, the employees say.

The phone call was part of a continuing cleanup that has led Wells Fargo to fire four foreign-exchange bankers and federal prosecutors to open their own investigation of the operation, people familiar with the matter have said.

“Wells Fargo remains committed to our foreign exchange business,” the bank said in a statement Monday. “If we find a problem, we fix it.” The bank said its foreign-exchange business is “under new management.”

The business is tiny compared with foreign-exchange operations at J.P. Morgan Chase & Co. and Citigroup Inc. but could become another huge headache for the San Francisco bank, still grappling with fallout from the sales-practices scandal in its retail operations. The scandal led to last year’s abrupt retirement of Wells Fargo’s chief executive, a $185 million regulatory settlement and numerous federal and state investigations, which are continuing.