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.