The nation’s second-largest mobile carrier will start sharing customers’ web and app data with advertisers on April 26, unless they opt out of the practice. The change was included in a recent privacy policy update.

The changes won’t apply to business or children’s phone lines, but the remainder of its 80 million customers (including those who have prepaid plans) could be affected. The company says the move is meant to deliver more relevant ads.

T-Mobile says it will mask subscribers’ identities and not share which websites they visit or apps they install, but some groups have still raised a red flag about the practice. Cybersecurity researchers have successfully traced anonymized data back to individuals in various instances.

Feeling uneasy about your cell phone company sharing that data? Opting out is easy, though the practice differs depending on which T-Mobile division you use.

T-Mobile

• Open the T-Mobile app and click the “More, Advertising & Analytics” tab.
• Scroll down to find “Use my data to make ads more relevant to me” and toggle it off.

You can also use the company’s website.

• Under My account, click Profile, then Privacy and Notifications.
• Find the “Advertising & Analytics” option and click that.
• Scroll down to find “Use my data to make ads more relevant to me” and toggle it off.

Sprint

• On the Sprint website, choose “My Account,” then preferences.
• Scroll to “All about my account” and select the “manage advertising and analytics” option.
• Find “Use my data to make ads more relevant to me” and toggle it off.

Metro

You’ll need to be using your phone to make changes, regardless of whether you use the app or the website:

• Go to the Account tab, then click through to Network and Location Settings.
• Scroll down to find “Use my data to make ads more relevant to me” and toggle it off.