Editor's note: Amy Gahran writes about mobile tech for CNN.com. She is a San Francisco Bay Area writer and media consultant whose blog, Contentious.com, explores how people communicate in the online age.

(CNN) -- Phone bills, especially cell phone bills, are notoriously complex and cryptic. This makes it easy to slip in new charges without consumers noticing -- including for services that the consumer never authorized.

"Cramming," the illegal practice of fraudulent phone bill charges, has been around for decades. But with the exploding growth of cell phones, it's gotten worse. On Monday, the FCC announced that it will propose new rules to increase phone bill transparency and disclosure in an effort to combat cramming.

This comes on the heels of another announcement that the FCC plans to fine four companies nearly $12 million for cramming: Main Street Telephone ($4.2 million), VoiceNet Telephone ($3 million), Cheap2Dial Telephone ($3 million), and Norristown Telephone ($1.5 million).

"We found that each of these companies was charging thousands of consumers for a type of long-distance service they never ordered or used, with each company billing consumers roughly $13-$15 at a time," said FCC Chairman Julius Genachowski. "This resulted in the apparent overcharging of consumers to the tune of about $8 million."

How widespread is cramming? Based on a recent expert survey, the FCC estimates that each year 15 to 20 million U.S. households have received "mystery fees" on their monthly landline phone bills. Only 5% of consumers who were impacted by a particular cramming company were aware of the monthly charges.

The FCC said its plans to increase phone bill transparency and disclosure apply to mobile as well as landline phone bills. And cramming does happen on cell phone bills. In Washington state, KXLY News recently reported on a class-action lawsuit filed against 23 text messaging companies allegedly engaged in cramming.

KXLY quoted Seattle attorney Toby Marshall, who explained: "When you get the text ... if you don't respond back and say stop [to opt out], they assume that you are now enrolled." The defendants in the suit worked with all the major cell phone carriers, KXLY notes.

How to protect yourself

For now, the FCC's cramming tip sheet recommends that consumers should closely scrutinize their phone bills every month to spot suspicious charges. The tip sheet gives examples of what cramming charges might look like.

However, this recommendation is exactly the problem -- especially for cell phone users.

Most cell phone bills are extremely complex, running dozens of pages long, if you really want to see everything you're getting charged for. So wireless customers often opt to receive only a printed bill summary via postal mail.

Furthermore, wireless customers are increasingly opting to receive no printed bill at all. Instead, they get a text or e-mail from the carrier when a new bill is due, and it's easy just to pay that amount without checking your bill details online. If the amount is within a few dollars of the bill's normal amount, customers probably would not bother researching their phone charges online.

If you suspect that your phone bill has been crammed, check back over previous bills to see how long you've been getting that charge. Then call the company that added those charges and tell them to cancel the service, remove the charges, and refund any previous charges.

It may not be easy to figure out who they are or how to contact them. Your phone company should be able to help you identify and contact the potential crammer.

If you believe you were tricked into signing up for a phone service you don't want, complain to your phone company and tell them to reverse the charges and refund previous charges. You can also file a complaint with the FCC.

The opinions expressed in this post are solely those of Amy Gahran.