Thousands of New Zealanders could have had their information published in phone books without their permission.
The problem was discovered after Spark and Yellow carried out an audit in November and found "several thousand" people had not had their requested listing preference followed, Spark spokesman Richard Llewellyn said.
"We have worked with Yellow and corrected most of the discrepancies, but some have been published in the printed version," he said.
The thousands discrepancies also extended to people who specified they wanted their details published being left out, and specifications around listed businesses with private personal information being omitted
People affected by the mistakes were sent letters by Spark, Llewellyn said, and the Office of the Privacy Commissioner had been informed.
Spark apologised to the people affected and advised people concerned about their immediate safety to contact police.
"From those letters 60 people have contacted us and 59 of them were people who wanted to be published in the phone book," Llewellyn said.
Llewellyn said he understood most people had "benign" reasons to not be listed, but in some cases it could be more serious.
He wouldn't say whether compensation would be possible for affected customers, but said Spark would be working on a "case by case basis".
Spark "wouldn't point fingers" and the cause of the privacy breach was being investigated, he said.
"They're our customers... and we will work to do right by them."
Yellow Chief Executive, Michael Boersen, said the company relied on telecommunications companies to provide it with information.
"It is up to telecommunications companies, like Spark, to provide us with accurate details of their customers' listing preferences for publication in our printed phone books, online at whitepages.co.nz and via 018 Directory Assistance.
"Yellow has worked closely with Spark to help correct the listing preferences of some of their customers since we became aware in November that some records did not match.
"We carefully follow the process for the confidential, unlisted or non-published requests provided to us, and we take this very seriously."
Llewellyn said Spark had encouraged other telecommunications to carry out audits on the information they shared with Yellow.
Llewellyn said Spark had encouraged other telecommunications to carry out audits of their information shared with Yellow.
Vodafone spokeswoman Andrea Brady said their customers had not been affected.
"We are aware that Spark has been dealing with it... We've had a look at out relationship with yellow and we don't think any of our customers have been affected by it. We have different processes and things in place," she said.
The Office of the Privacy Commissioner was aware of the breach, spokesman Charles Mabbett said.
"Obviously the most concerning aspect of this is the number of customers in the printed phone book without their wanting to be."
Mabbett said Spark had followed "best practice for a data breach" by establishing an 0800 number to deal with concerned customers.
If anyone felt harmed by the privacy breach they should lay a complaint with the Office, Mabbett said.