The chair of the U.S. Senate's antitrust panel sent a letter to four top mobile phone companies on Tuesday asking them to explain what he said were a doubling in the price of text messages in three years. Sen. Herb Kohl, chair of the antitrust subcommittee and a Wisconsin Democrat, wrote to Verizon Wireless, AT&T, Sprint Nextel and T-Mobile to express concern about "what appear to be sharply rising rates your companies have charged to wireless phone customers for text messaging."

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