Federal regulators are asking Verizon Wireless to explain why it doubled to $350 the fee customers pay for terminating smart-phone contracts.

The Federal Communications Commission made the request in a letter Friday to the largest U.S. mobile telephone provider.

Sen. Amy Klobuchar, D-Minn., and three other U.S. senators introduced legislation Thursday to limit the fees. Verizon Wireless on Nov. 15 raised the fee from $175 for customers purchasing a smart phone with a one- or two-year service agreement, the senators said in a news release.

Customers can pay full price for mobile phones rather than pay the fees, which help people afford wireless Internet connections, Verizon Wireless spokesman Jeffrey Nelson said Friday.

"If you stay with your contract, you don't pay a fee at all," Nelson said.

The fees affect phones such as Research In Motion BlackBerrys and Motorola Inc. Droid devices powered by a Google Inc. operating system, Verizon Wireless spokesman Jim Gerace said Nov. 5.

The Basking Ridge, N.J.-based company is trying to make the termination fees correspond with the prices of the phones, which Verizon Wireless subsidizes, Gerace said.

Klobuchar's legislation would forbid companies from charging a termination fee higher than the discount on the telephone offered with the contract. The bill would require "clear and conspicuous disclosure" of fees that would need to be pro-rated so consumers owe half after one year of a two-year contract, the senators said in the statement.

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