It's called the ""perma-cookie." As we told you last week, Verizon Wireless has embraced this privacy-busting digital marker, which could give marketers a way to track what you do online whether you want to be tracked or not. Basically, the U.S. wireless carrier is slipping the cookie onto the smartphones that tap the net via its celluar and data network, and most phones owners don't realize it.

Privacy and networking experts really hate the thing—a little strings of data that get inserted into our unencrypted web traffic—because it undermines the way the internet is supposed to work and it wrest control of our online experience away from us. It's viewable by any website you visit, and it could be used by advertising networks to build elaborate profiles of everything we do online.

In the wake of the revelations about Verizon, many have wondered if other carriers are doing the same thing. The answer is "Yes"—and "No."

AT&T uses them too. Although the company says it's only testing them out right now. But this week, we also reached out to T-Mobile and Sprint this week, and they told us, no, they do not use them.

That's in line with what the perma-cookie testers we know are seeing. University of California, Berkeley researcher Nicholas Weaver has set up a website where users can check for the cookies, and he says he hasn't seen anything from Sprint or T-Mobile customers. Neither has Kenneth White, a security researcher who runs another website that checks for the cookies. Weaver and other researchers say that according to their data, there are only a handful of companies worldwide that are known to do this. And that puts the spotlight firmly on Verizon and AT&T.

Why are the nation's two largest carriers doing something that's so unfriendly to consumers?

Part of it may be simple Google envy. The big carriers see the big money being made by Google's advertising business and they wonder why they shouldn't have a piece of that. "Part of the answer may lie in Maciej Ceglowski's notion of "investor storytime"," says Jacob Hoffman-Andrews, the Electronic Frontier Foundation researcher who coined the term perma-cookie.

"The promise of advertising is that 'if only we could track users better we'd make so much money,'" he says. "But the first step is adding the tracking, and then the next step is ramping up the advertising volume to actually make the money. It's possible Verizon is in that middle period between implementing the tracking and proving it makes money."

In that scenario, Verizon and AT&T have the corporate might to install the "header enrichment" hardware that will insert the extra perma-cookie data. And if consumers and government regulators let it pass, then we may see a new breed of telecommunications companies that do much more than sell services to businesses and consumers. These companies will also essentially sell their consumers to advertisers.

How that will change things is unclear. It seems naive to expect Verizon and AT&T to copy Google's model and offer us extremely low-cost or free service in exchange for our privacy. More likely, consumers will be left with the worst of both worlds: expensive data plans that track them and then sell that information to all bidders.

But until that happens, there's still something that privacy minded users can do: Switch to Sprint or T-Mobile.