Traditionally, the practice of selling customer data to marketing agencies was limited online heavies but, apparently, that is no longer the case.

Now wireless carriers are starting starting to sell customer data –– a boon to marketing agencies and a concern to privacy advocates.

A report by The Wall Street Journal found that big phone companies and their mobile operators are now selling the mounds of data they've accumulated on customers. That data typically includes information on a mobile customer's movements and mobile web browsing habits.

By tracking the location and habits of customers, marketers can easily adjust their ads to cater to customers fitting a particular demographic.

One major carrier, Verizon Wireless, is already selling data through its industry product, Precision Market Insights, which launched in late 2012. Verizon said it typically sells data to subscribers like malls, billboard owners and stadiums.

Such deals aren't limited to the U.S., who's citizenry is becoming increasingly comfortable with data collection. European mobile operators, too, are experimenting with selling customer data. The German giant SAP is partnering with European wireless agencies to collect, package, and sell user data to marketers. Part of the profits return to the wireless operators.

Both Verizon and SAP said the identities of customers are protected through a process rendering them as anonymous to marketers. And customer data on location and web browsing isn't sold on an individual basis: information on customers habits is sold is sold in groups.

But despite those standards, some privacy advocates are still worried, saying derived profits could give wireless operators a reason to closely track user habits. That, Chris Soghoian, a privacy specialist at the American Civil Liberties Union, tells The Journal, could broaden the data accessible to law enforcement through subpoenas.

The move additionally marks a shift in the consumer and mobile network operator relationship. Instead of merely selling customers the means of remotely transferring data, networks are now mining customers for information they can later sell - a la Facebook and Google.

Verizon customers, however, opt out of being tracked by logging into Verizon's website and adjusting their settings. Government officials and corporations, by default, aren't tracked.