T-Mobile notes that personally identifying data is not shared and that users are given unique advertising identifiers to serve relevant ads to subscribers:
For most of how we use your data, nothing’s changed. However, starting April 26, 2021, T‑Mobile will begin a new program that uses some data we have about you, including information we learn from your web and device usage data (like the apps installed on your device) and interactions with our products and services for our own and 3rd party advertising, unless you tell us not to.
This means that T-Mobile subscribers have to proactively opt-out if they don’t want their data shared for third-party ads. T-Mobile also specifies that certain data like precise location data won’t be shared unless users allow it. This also will not apply to businesses or childrens’ lines.
This is hardly different from what the best carriers in the U.S. do. Still, it is a fairly questionable move for the “Uncarrier” and second-largest U.S. mobile network by subscribers, particularly in the wake of WhatsApp’s privacy drama with Facebook. This also changes things for Sprint customers since they previously had to opt-in to these kinds of data sharing practices, something that T-Mobile also highlights.
This program changes the way Sprint offered choices for sharing in the past, as this data was previously used only if you indicated that it was OK with you first. We may occasionally ask you to confirm your choices.
The new policy change affects Metro by T-Mobile customers as well. Fortunately, T-Mobile makes it fairly easy for subscribers to opt-out of the service by signing into your account and changing the settings in your privacy tools. Sprint and Metro by T-Mobile subscribers can also visit their respective links in T-Mobile’s updated privacy notice to change their settings.