The degree to which mobile carriers sell your personal data to third-party companies, allowing targeted ads based on your data, is rather tricky to ascertain from their privacy policies.

If you want to play safe, however, all three of the largest US carriers offer opt-out options…

CNET did the legwork, digging into the privacy policies of AT&T, T-Mobile, and Verizon to see what each had to say and how to opt-out.

The site shows opt-out instructions for all three carriers. For example, for AT&T:

AT&T, T-Mobile, and Verizon track your location, web activity and app usage. They also collect Social Security numbers, photographs and social media activity. AT&T even collects users’ biometric data like fingerprints, according to the carrier’s privacy policy — though only “until the initial purpose for collecting it has been satisfied or within three years of the individual’s last interaction with AT&T, whichever occurs first.”

Carriers use personal information to improve customers’ experience, according to their privacy policies, and to sell targeted ads. But you can stop your data from being sold, and some carriers even offer ways to delete the data they have already collected on you.

The reason for the different processes in California and Nevada is due to their more stringent state-wide privacy laws.

1. Log into your AT&T account from a browser. 

2. Go to this control panel. 

3. At the bottom, there’s a section called Control How We Use Your Data. Click through each option and toggle On or Off for phone numbers on your account. Each phone number can opt in or out and it won’t have an affect on the other phone numbers on your account.

Customers who live in California and Nevada have it easy. 

1. Fill out this Do Not Sell My Data form in a browser.

2. Click Do Not Sell My Data or Do Not Sell My Data Except to AT&T Companies at the bottom of the page.

AT&T customers in California can also request most of their data be deleted.

Check out the piece for similar instructions for T-Mobile and Verizon.

Image: UniqueKiosk