Tuesday, Wisconsin Democrat Herb Kohl, chair of the Judiciary Subcommittee on Antitrust, Competition Policy and Consumer Rights, sent a letter to the four major phone companies in the United States, inquiring about the doubling in text messaging prices.

Verizon Wireless, AT&T Incorporated, Sprint Nextel Corporation and T-Mobile were asked to offer an explanation to the fact that, over the last three years, the charge for text messaging via wireless phones has increased by 100%. In 2005, the price was 10 cents, while currently, all four carriers charge 20 cents for the Short Messaging Service. Senator Herb Kohl also expressed his concern about the fact that the change occurred at nearly the same time and that each company raised their price with 10 cents, thus decreasing the competition.

The four mobile phone companies, whose services cater for the needs of 90% of the United States consumers, have until October 6 to provide the Senator with the requested explanation and with comparable pricing data for voice calling, email and wireless Internet, also.

Of the four, Sprint was the first carrier to hike the price, the others shortly following its lead until August 29, when T-Mobile’s 33% increase came into effect, the company being the last to raise the fee on its SMS.

AT&T Mobility LLC was founded in 2000 as a joint venture of SBC Communications and BellSouth and it currently prides in having the largest number of subscribers throughout the United States. It is followed by Verizon Wireless, a network that has a total number of 68.7 million subscribers, 16.7 million more than Sprint Nextel Corporation. T-Mobile is a group of mobile phone corporate subsidiaries that operate networks in both Europe and the United States.