AT&T has stopped stamping unique ID numbers on its customers, which allowed websites and ad networks to track them across the web. The telco promised to offer an opt-out switch if the system returns.

"We have completed testing of the numeric code that would be part of any new mobile Relevant Advertising program we may launch," AT&T said in a statement to The Register today.

"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 device. Customer trust is important to us, and customers have choices about how we use their information."

Some mobile network providers are getting a little sick of customers binning standard web cookies that track their movements online, depriving the poor telcos of much-needed advertising revenue and profile-building capabilities for targeted ads.