T-Mobile customers will see a major privacy policy change next month. Unless users actively opt out, they will be enrolled by default.

According to T-Mobile's privacy notice posted online last month, beginning April 26, "Your web and device usage data, including apps installed on your device, and information from your interactions with our products and services, . use some data we have about you," the new data collection program will begin.

This personal information is more detailed than that collected by AT&T and Verizon's basic data collection programs and could be used by third-party advertisers as well as T-Mobile's own advertising.

According to the Wall Street Journal, AT&T and Verizon have other data collection programs that collect more detailed information about their customers and share it with third parties.

The difference is that users must choose to participate in those programs. Sprint, which merged last year and is now part of T-Mobile, had a similar program, but it too was opt-in.

T-Mobile's new program is opt-out, meaning that data will be collected and shared unless users take active steps to remove themselves.

"We've heard a lot of people say they prefer more relevant ads, so we're making this [opt-out] setting the default," a T-Mobile spokesperson told The Journal.

The Journal suggested that the change may be more for former Sprint customers than for longtime customers of T-Mobile or its prepaid subsidiary, Metro by T-Mobile.

We asked T-Mobile for further clarification. The response was:

"This new change will allow us to serve ads based on customer interests. This new change will allow ad serving based on customer interests."

We have updated our customer privacy choices to match how we will be using the data.

T-Mobile's privacy notice clearly states that by the time this detailed data is passed to a third party, it "will not be tied to your name or information that directly identifies you" and "instead will be tied to your mobile ad identifier or another unique identifier."

That sounds good, but any privacy expert knows that ad IDs in smartphone apps can be easily and quickly traced back to an individual.

Aaron McKee, an attorney at the Electronic Frontier Foundation, told The Journal, "This kind of data is so personal and revealing that it's trivial to tie that de-identified information to you."

At the 2018 hacker conference DerbyCon, Chicago-based researcher Mark Milhouse showed that a $100 smartphone ad purchase makes it possible to track the physical location of any individual with just an ad ID.

Ars Technica dug up T-Mobile's web page and specified the types of data that "we and others may collect and use for advertising and analytics."

The data set includes:

It is not clear which will be used for analysis, which for advertising, and which for both.

According to The Journal, to opt out of T-Mobile's new data collection program, T-Mobile customers can use the T-Mobile app: under the "More" tab, look for "Advertising and Analytics" and click on " Use my data to make ads more relevant to me" option off.

T-Mobile users can go to the T-Mobile website, go to "My Account" > "Profile" > "Privacy and Notifications" > "Advertising and Analytics" and turn off the "Use my data to make ads more relevant to me" can be turned off.

Former Sprint users should log into their account on the Sprint website, go into their account settings, find Advertising and Analytics settings, and switch off "Use my data to make ads more relevant to me."

Metro users can use either the MyMetro app or the Metro website, but the website must be accessed through a mobile browser. In both cases, go to Account > Network and Location Settings and turn off "Use my data to make ads more relevant to me."