However, policing the hundreds of companies in the online tracking business was likely to be a difficult task for Verizon.
And so, on Monday, Verizon followed in the footsteps of AT&T, which had already declared in November that it would stop inserting the hidden undeletable number in its users' Web traffic.
In a statement emailed to reporters on Friday, Verizon said, "We have begun working to expand the opt-out to include the identifier referred to as the UIDH, and expect that to be available soon."
Previously, users who opted out from Verizon's program were told that information about their demographics and Web browsing behavior would no longer be shared with advertisers, but that the tracking number would still be attached to their traffic.
For more coverage, read ProPublica's previous reporting on Verizon's indestructible tracking and how one company used the tool to create zombie cookies.
"The zombie cookies will allow AOL to "acquire demographic data on users" while simultaneously using their own advertising network to track user browsing history, use pf apps on smartphones and their geo-location coordinates. Earlier this year, ProPublica released a report regarding the advertising company called Turn and their zombie cookies that are used by large tech firms to "come back to life" even after users have deleted them. In the ProPublica report, it was revealed that Turn is "taking advantage of a hidden undeletable number that Verizon uses to monitor customers' habits on their smartphones and tablets" by respawning those "tracking cookies that users have deleted." Called unique identifier headers (UIDHs), or perma-cookies, this sneaky monitoring of customers is used "to help marketers create more targeted ads based on their customers' unique browsing habits." In 2012, UIDHs were used by Verizon to provide a way for advertisers with "demographic and third-party interest-based segments" to help them deliver "relevant ads" based on mobile devices' unique identifiers. Shockingly, more than 100 million Verizon customers were affected by this snooping by the corporation, tracking individual customer usage and reporting the findings to the federal government and advertising corporations."