It was a huge problem back in the landline phone days, but mobile phones are crammers' new target, and you may not even know it.
When a text offering Wen Chao a deal to download mobile content for $9.99 a month appeared on her phone, she ignored it. But when a second one popped up from the same sender, she opened it.
"I thought, 'Oh, maybe it's from a friend whose number I don't have in the contacts,'" said Chao.
She called her mobile phone carrier to block future mystery messages, and got some shocking news.
"I was told that by the act of opening the text message I had consented to what they were trying to sell me," said Chao.
That $9.99 charge appears on her bill as "premium messaging."
Industry experts say charges like this are costing consumers more than $600 million each year. The Federal Trade Commission is cracking down and recently filed its very first case against a company for mobile cramming.
"We're very concerned about what we see as a growing number of complaints," said FTC attorney Duane Pozza.
The FTC says it has received thousands of complaints -- but they say you could be a victim and not even know it.
"Many consumers overlook the charges on their phone bill, so the complaints that we see really are just the tip of the iceberg," said Pozza.
Consumers miss these charges, the feds say, because they appear on mobile-phone bills as innocuous-sounding fees like "standard rate plan," "member fee" and voicemail.
So why don't cellphone companies that also profit when third-party companies charge your bill make these fees more prominent?
"If you have thousands of different kinds of services available, that wouldn't be practical for a billing system to be able to specifically list those thousands. That would be a pretty expensive proposition," said John Walls, CTIA-The Wireless Association.
There's no federal law giving you the right to dispute questionable mobile phone charges like there is with your credit card, and it's a carrier-by-carrier decision.
"You really need to be vigilant when you pay your bills," said Wen Chao.
The Federal Communications Commission (FCC) is considering requiring mobile carriers make third-party charges more obvious on phone bills.
In the meantime, if you're concerned about being crammed, you can call your cellphone company and ask them to block any third-party charges from being billed to your account.