AT&T's "experiment" with phone-tracking tags that gave users unique identifiers, even if the users had opted out of mobile ad-tracking services, is now over, according to the company.

The controversial tracking program and others like it had been criticized by privacy advocates who argued that tracking user information even if users opted out of the tracking was not fair.

"We have completed testing of the numeric code that would be part of any new mobile Relevant Advertising program we may launch," Emily J. Edmonds, director of communications for AT&T, told eWEEK Nov. 17 in an email reply to an inquiry about the practice. "Any new program we would offer would maintain our fundamental commitment to customer privacy. Customers will be able to opt out of the ad program and not have the numeric code inserted on their devices. Customer trust is important to us, and customers have choices about how we use their information."

Edmonds wrote that the testing was built around "a numeric code that changes every 24 hours on mobile devices as an important protection against unauthorized tracking. It would be used to help serve ads on an anonymous basis. It plays a similar role to a cookie in online advertising. Some people have seen this code from our testing when they visit test sites."

That testing has now been completed, and the experiment "has been phased off of our network," she added.

The tracking tags, which were also known as "perma-cookies," had allowed Internet sites to track a specific mobile phone and create a database of information about what the user of the phone was doing, such as looking for sports scores or searching for restaurants or shops, according to an earlier eWEEK report.

The initial details of the AT&T tracking were revealed recently by the non-profit journalism group Pro Publica, which discovered that Twitter is making use of this information from Verizon wireless phones as a way to deliver advertising.

In October, Verizon denied that it shared the identities of the affected users through its own controversial mobile advertising program, according to an earlier eWEEK report. Verizon Wireless began including Unique Identifying Headers (UIDH) in the address information of incoming Internet data requests from Verizon customers about two years ago.