Criminals have been taking out O2 phone contracts by stealing the identities of innocent Brits, leaving many with huge bills.
The issue has even wrecked customers' credit scores in the time it takes for the telecoms firm to investigate their complaints.
Criminals use this data to pretend to be real customers and get through tough security and credit checks.
The fraudsters can use the stolen information to order phones, all without the victim's knowledge.
One Mirror reader, Susan, said she only found out about the issue when she got a message from her credit checking firm Clearscore on November 14 last year.
She found an O2 phone contract and a loan, neither of which she recognised - despite both being in her name.
"I have never had a phone contract in my life or any dealings with O2," she said.
Soon afterwards she told O2 about the issue, as well as her bank.
Fortunately no money was taken from her bank account, but that also meant her case was not taken as seriously by her bank.
She said she was "pulling my hair out" over the issue, which was eventually sorted out on February 10 with a £30 payment from O2 to say sorry.
This is Money reports that several of its readers have reported the same issue.
In one case a 76-year-old woman was chased by O2 for payment arrears on an iPhone and iPad - which she never paid for.
Another, Julie McCue, 37, got two O2 bills totalling £918.
Often the targets of these frauds have had their details stolen or sold to fraudsters.
Cybersecurity expert Jake Moore told This is Money: "Once scammers have identified a weakness in a company’s security procedure, they will continue to abuse it until that door is closed."
An O2 spokesperson said: “At Virgin Media O2 we take the security of our customers incredibly seriously and protecting them from fraud is a priority.
"We have robust processes in place and always act quickly when we see new patterns of fraudulent activity emerging to help keep our customers safe.
"Criminals committing fraud are increasingly sophisticated in how they scam people and defraud companies, so to tackle this we are continuing to introduce additional security checks, leading cross-industry initiatives and working with government and law enforcement to protect our customers.”
Last week The Mirror reported that O2 and Virgin Mobile customers will see their bills hiked by as much as £48 a year as the cost of living crisis continues to squeeze households.
Both networks, which merged last year as part of a £31billion tie-up, will raise prices for some pay monthly customers by up to 11.7%.
This is calculated based on the retail price index (RPI) rate of inflation - which now stands at 7.8% - plus 3.9%.
The price rises will come into force from April 1, 2022 - but exactly how much more will be added to your bill will depend on which network you're with.
It also varies depending on what type of contract you’re on and when you joined O2 or Virgin.
O2 customers who joined after March 25 last year will face the full 11.7% price hike - adding as much as £48 a year to your bill.
But if you joined before March 25 last year, your price increase will be capped at the rate of RPI inflation, which is 7.8%.
