WASHINGTON, July 12 (Reuters) – Telephone companies would have to make third-party charges on landline phone bills clear and distinct to help people detect unauthorized fees under rules proposed on Tuesday by U.S. communications regulators.

The Federal Communications Commission unanimously proposed rules to against so-called mystery fees and “cramming,” the illegal placement of extra charges on phone bills.

Cramming may affect up to 20 million Americans, the FCC estimates, with only one person in 20 realizing that they are being charged for unwanted services.

“We’re sending a clear message: if you charge consumers for something they didn’t request, you’ll be discovered and you’ll be punished,” FCC Chairman Julius Genachowski said.

The FCC’s proposal would require landline phone companies to tell people about any blocking services they offer and denote charges from third parties in a separate section of the bill from the telephone company’s charges.

The rules also would require the FCC’s contact information to appear on all landline and wireless phone companies’ bills and websites with a notice that customers can file complaints with the agency.

Cramming generally adds anywhere from $1.99 to $19.99 a month to a phone bill. The charges can originate from the phone companies or from third parties for services such as horoscopes, yoga lessons and diet plans.