We'll try to make this quick, but the overall topic is anything but limited.

First, nobody is safe when it comes to being a victim. Fraud happens in both fixed and mobile networks of all technologies. There is no such thing as a technology that cannot be compromised -- those intent on fraud will find a way. They always do. As one vulnerability is closed, a creative criminal will find another.
Sometimes the telephone company (seldom the BIG ones, just those pesky upstarts) gets you: One method is "cramming," the addition of charges to your phone bill that cover services you didn't order and fees for things that you may have been led to believe were free. Then there's "slamming," which is when your long distance or DSL carrier is changed -- usually via a telemarketer who you said NO to. You suddenly find yourself doing business with "Bingo Crapshoot's Pool Hall and Long Distance Provider" and their charges are as outrageous as their name.
If you own a business that uses a PBX system, a fraudster can call into the company and then ask to be transferred to an outside toll number. Most operators are smart enough to not let this happen, but once in a while a good line of patter will sneak through. The most common ruses are system tests, although collusion with inside employees will also work. ("Hey, Joe. Patch me through to 1-900-Hot-Sexy off your company line and I'll buy you a beer on Friday.")
Beware of 809 area codes on call-backs -- or, in fact, any area code that you do not recognize. 809 goes to the offshore Atlantic Islands, those that are not subject to US laws. The numbers can be set up to bill in a fashion similar to a 1-900 number, and by the time you ask for whomever allegedly left the urgent message you can be on the hook for hundreds of dollars.

Fraud can be external or internal. It may come from the customer base or it may come from inside of the company.

Within communications, fraud has two main categories. Revenue Fraud involves money to the person perpetrating the fraud. Non-revenue fraud is motivated by more personal objectives, and it often involves helping out a friend (even though the intent is criminal) or just defeating the system for the sheer thrill of it.

In a few cases, it is the criminal network who is perpetrating the fraud. For instance, a smuggling ring may want to understandably avoid surveillance and the threat of phone-tapping -- so they figure out a backdoor into the telephone network and steal communications services. This may be day-by-day, hour-by-hour, or even call by call. Most cases, however, are motivated by pure financial greed.

Recently, AT&T was attacked by scammers who made off with the records of 2500 customers. These fraudsters are likely selling the information to other crooks or, perhaps, to one of the those data mining companies that are everywhere on the net where you pay the price for the information that was illegally obtained.

There are many kinds of Phone Revenue Fraud:

  • Call Selling -- services are stolen and then sold at a discount. The buyers may be fully aware that they are purchasing "hot minutes" or they may be clueless.
  • Putting unintended calls through to a revenue 900 line.
  • Subscription fraud: obtaining service by using a false identity. In some cases, the person is unable to get credit in his own name, so even if he uses another person's identity, he pays his bills because he wants to keep the line. In other cases, however, it is referred to as a "No Intention To Pay" fraud because the criminal will simply obtain line after line after line, never once settling a bill. This one is commonly used in the drug trade where numbers are constantly changed within the course of doing business. 
  • Surfing: This is thievery of services on a call by call basis. It may involve cloning, obtaining calling card details from an innocent third party.
  • Ghosting -- a techie way of deceiving the network and getting free or discounted calls. Past scams have included suppressing the answer signal so that no charge is generated for the call, getting cash refunds on bills, stealing customer information to use in future identity theft schemes, etc.
  • It's nearly impossibly to quantify the amount that is stolen from companies by fraudsters, but it is not impossible to determine who pays the eventual price of this thievery. YOU do.

Consider a single reported case:

A night janitor picked up an office telephone of a large corporation and dialed a 900 number. He left the phone off the hook during a Saturday afternoon janitorial visit. From that time until Monday morning when the employee arrived to work, the line was building up charges at a rate or $8 per minute. The employee thought nothing of the line being off the hook, assuming that it was inadvertently knocked over, and hung the phone up. The charges? $480 per hour for more than 40 hours. Not only did the 900 operator benefit from this call, but the phone company simply shuffled the $20,000 into their uncollectible line item and dropped the charge from the customer's bill. To the phone company, uncollectibles are a cost of doing business, one that is passed off to the consumer block.

Who is in that block? YOU are.