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But Verizon is still using them:
#^AT&T dropping cellphone tracking codes – The Spokesman-Review http://www.spokesman.com/stories/2014/nov/15/att-dropping-cellphone-tracking-codes/

WASHINGTON – AT&T Mobility, the nation’s second-largest cellular provider, said Friday it’s no longer attaching hidden Internet tracking codes to data transmitted from its users’ smartphones. The practice made it nearly impossible to shield its subscribers’ identities online. The change by AT&T essentially removes a hidden string of letters and numbers that are passed along to websites that a consumer visits. It can be used to track subscribers across the Internet, a lucrative data-mining opportunity for advertisers that could still reveal users’ identities based on their browsing habits.