Today's cell phones are a marvel of technology. You can surf the Web and download music, movies, ring tones and even books.

But unless you're careful, you can ring up a whopping bill.

Helen Schwartz got a bill for her son's cell phone that nearly knocked her off her feet. The total amount charged for just five weeks of service was $ 5,174.

"It was so outrageous that I didn't even know where to start," she said.

Back in March, Schwartz bought her son Daniel a phone from AT&T Wireless.

When she signed up for the service, she did not buy an Internet package and did not pay to block Internet access because her son had agreed not to go online.

But as any parent knows, 13 year olds don't always follow the rules. Daniel went online to download a couple of ring tones and to check his e-mail.

"I checked my mail and I got off it," he said.

But it wasn't so much what Daniel did as much as what he didn't do that rang up the charge.

"I didn't sign out, so it just kept going and going," he said.

Daniel says his phone could have been hooked up to the Internet for several days with the meter running the entire time because he exited the screen without signing out.

He says he was shocked when his mom showed him the bill.

"I couldn't believe it," he said.

Without an Internet package, time spent online by the cell phone user is billed at 1 cent per kilobyte, which resulted in that staggering bill. But in this case, it was an honest mistake.

Jenny Dougherty got stung by a download scammer.

"It said 'free ring tone. No subscription needed,'" she said.

So she texted in the four-digit numbers that identifies her phone number. Before long, she started getting junk text messages.

"One said 'health care' and one said 'payday loans,'" Dougherty said.

Even worse, she got charged for each message.

Dougherty ended up with more than $100 worth of bogus charges, many from somewhere outside the country. She complained to the phone company and was told no one there could help her.

"(The company said) I have to pay the bill and if I want any of the charges reversed, I have to try to contact these companies myself," she said. "And I tried and half of them are disconnected."

In order to avoid future charges, Dougherty had to order a block for every phone number that sent a text message to her cell phone.

As for Daniel, he was in luck.

AT&T agreed to reversed most of the charges, and the bill was reduced to $167.

Wireless companies will often make adjustments to the bill the first time there's a problem, but there's no guarantee. So you need to make sure you choose the right plan. If the user will be texting or going online, you want a text or data plan.

If the phone is for your kid, find out about parental controls that let you block or limit access to expensive services. There is a charge for this, but it could save you money in the long run.