Piggybacking on Verizon's announcement Sunday that it would give $90 million in refunds to customers, Sen. Amy Klobuchar wrote a letter to the Federal Communications Commission urging "robust oversight" of the wireless industry's billing practices.

Klobuchar, along with Democratic Sens. John Kerry of Massachusetts and Mark Begich of Alaska, wrote to the FCC Tuesday citing the Verizon case and a recent FCC survey that found one in six wireless customers have experienced "bill shock" over unexpected charges.

The letter said the FCC's involvement in the Verizon dispute led to the company agreeing to pay $90 million back to 15 million customers. The refunds stem from Verizon charging customers for data services they were not using.

"Before the FCC's engagement, many customers were left with no choice but to pay for services they simply did not use," the senators wrote.

In December 2009, Klobuchar and Begich introduced legislation attempting to curb high early termination fees attached to many cell phone contracts.