A Senate Democrat is calling on the nation's top four cellular service providers explain rising text messaging rates, up from 10 cents a message to more than 20 cents, according to the Associated Press.

The story states that Wisconsin Democrat Herb Kohl, a member of the Senate Judiciary Committee, told Verizon Wireless, AT&T Inc., Sprint Nextel Corp. and T-Mobile in a letter Tuesday

"... that he is concerned that rising text messaging rates reflect decreasing competition in the wireless business."

This increase, he said, "does not appear to be justified by rising costs in delivering text messages," which are small data files that are inexpensive for carriers to transmit.

Kohl said he is particularly concerned that all four of the companies appear to have adopted identical price increases at nearly the same time. "This conduct is hardly consistent with the vigorous price competition we hope to see in a competitive marketplace," he wrote.

That's the rub when it comes to the price increases: