Both Verizon and AT&T appear to be engaging in some unsavory customer tracking techniques, using unique identifying numbers to deliver targeted advertisements to customers in what's called "Relevant Advertising." As outlined by Wired, Verizon is altering the web traffic of its customers by inserting a Unique Identifier Header or UIDH, a temporary serial number that lets advertisers identify Verizon users on the web.
According to Jacob Hoffman-Andrews of the Electronic Frontier Foundation, the UIDH serves as a "perma-cookie" that can be read by any web server to "build a profile" of internet habits. Verizon users cannot turn off the UIDH, but opting out of the Relevant Mobile Advertising Program prevents the information from being used to create targeted ads.