One of the biggest problems that consumers have faced with mobile phone billing in recent years is that there's really no way of independently measuring the amount of data that's being consumed by a mobile Web session. Consumers are at the mercy of the wireless carriers and have put their trust in these providers to accurately bill them.

Now, AT&T finds itself at the center of a class action lawsuit that alleges that the provider's bills "systematically overstate the amount of data used on each data transaction." Granted, the overstatement that's being alleged is small - somewhere in the range of 7-14 percent monthly, according to a post on the Electronista blog.

What's especially telling is how a consulting firm that was hired by the lawyers of the plaintiff conducted its own test of the data billing. Instead of using data and trying to measure it independently for comparison against the bill, the consultant did the exact opposite. The firm bought a new iPhone and immediately turned off all push notifications and location services, made sure that no apps or email accounts were active and then left the iPhone idle for 10 days.

AT&T billed the account for 2,292 kilobytes of data over 35 transactions.

Granted, that's not a ton of data. But, multiply any overage charges by the number of iPhones that are being used on the AT&T network and it could have some impact on AT&T's data revenue numbers, the complaint alleged, comparing it the practice to that of rigged gas pumps that only pump nine-tenths of a gallon of gas but record a full gallon.

AT&T responded to the allegations in a statement to the blog. It read:

Transparent and accurate billing is a top priority for AT&T. In fact, we've created tools that let our customers check their voice and data usage at any time during their billing cycle to help eliminate bill surprises. We have only recently learned of the complaint, but I can tell you that we intend to defend ourselves vigorously.

AT&T previously offered an unlimited data usage package but switched to tiered packages with caps. Shortly after Verizon announced that the iPhone - which is hitting that network on Feb. 10 - would be eligible for an unlimited data package, AT&T reportedly has been allowing some of its iPhone customers to switch back to an unlimited plan on a case-by-case basis. The company has not confirmed that.

Regardless, this latest complaint illustrates how consumers remain confused by billing for data consumption. If I wanted to carry a stopwatch around, I could literally track and log all incoming and outgoing voice minutes used on my mobile phone over a given time period. But there is no easy way for consumers - or even independent firms - to accurately measure the data consumption.

And until there is, I wouldn't be surprised to hear more squawking about the billing for data. As the usage increases, the disconnect between carrier and customer potentially can only get worse.

I'm waiting for a copy of the actual complaint to be sent my way and will update this post with an excerpt, as well as a copy of the court filing, when it arrives.