The U.S.'s top wireless carrier is handing out persistent cookies that serve as unique identifiers for each mobile devices it tracks, but Verizon isn't the only telecom treating ad agencies with the old trick.

Known as unique identifier header, or UIDH, each of the persistent cookies uses a unique sequence of 50 characters to identify the mobile devices on which it is stored.

The UIDHs, one of many different types of cookies, help to identify mobile devices as the smartphones and tablets connected to difference websites. And it's that web history of sites visited and time spent on them that enables advertisers to deliver targeted ads to mobile device users.

Verizon is said to have been using UIDH cookies for the last two years. Electronic Frontier Foundation technologist Jacob Hoffman-Andrews is calling for an end to the wireless carrier's tracking tactics.

"ISPs are trusted connectors of users and they shouldn't be modifying our traffic on its way to the Internet," says Hoffman-Andrews.

A Verizon spokesperson says there's no way for the company to turn off UIDH, though the company's customers can opt out of receiving targeted ads. However, opting out of targeted ads doesn't stop Verizon from sharing the UIDH with websites to which users connect.

In a May pitch to advertisers, Verizon announced its Precision Market Insights platform for mobile devices. Verizon said its position at the top of the U.S. mobile market puts it in a unique position to help marketers reach consumers.

"Today, I might look for sporting goods at a specific retailer, and the next time I visit a site with space for advertising -- let's say a news site -- I may see an ad for that retailer," stated a Verizon official in May. "Now, using Precision's solutions, advertisers may deliver that ad for the same retailer when I visit a mobile website that has space for advertising."

AT&T doesn't have a targeted ad program rolled out in full right now, but the company is considering it, according to a statement the wireless carrier recently released. AT&T says it is testing programs that would maintain the level of privacy its users expect.

"For instance, we are testing a numeric code that changes every 24 hours on mobile devices to use in programs where we serve ads to the mobile device," stated AT&T. "This daily rotation on the numeric code would help protect the privacy of our customers. Customers also could opt out of any future AT&T program that might use this numeric code."

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