AT&T says it has stopped its controversial practice of adding a hidden, undeletable tracking number to its mobile customers' Internet activity.

"It has been phased off our network," said Emily J. Edmonds, an AT&T spokeswoman.

The move comes after AT&T and Verizon received a slew of critical news coverage for inserting tracking numbers into their subscribers' Internet activity, even after users opted out. Last month, ProPublica reported that Twitter's mobile advertising unit was enabling its clients to use the Verizon identifier. The tracking numbers can be used by sites to build a dossier about a person's behavior on mobile devices – including which apps they use, what sites they visit and for how long.

The controversial type of tracking is used to monitor users' behavior on their mobile devices where traditional tracking cookies are not as effective. The way it works is that a telecommunications carrier inserts a uniquely identifying number into all the Web traffic that transmits from a users' phone.

AT&T said it used the tracking numbers as part of a test, which it has now completed.

Edmonds said AT&T may still launch a program to sell data collected by its tracking number, but that if and when it does, "customers will be able to opt out of the ad program and not have the numeric code inserted on their device."

A Verizon spokeswoman says its tracking program is still continuing, but added "as with any program, we're constantly evaluating."

Verizon uses the tracking number to identify the users' behavior and offer advertisers insights about users gleaned from that data. Verizon says the data it sells is not tied to a users' identity. "None of the data that is used in the program is personally-identifiable," the company said when it updated its privacy policy in 2012.

Verizon offers its customers an opportunity to opt out of the program. But opting out doesn't remove the tracking ID.