Budget MVNO Gifgaff faces a fresh challenge from Ofcom over claims it provided the regulator "inaccurate information" during a probe into its billing cock-up.
The UK comms prober opened the investigation last year and said the mobile virtual network operator may have sent inaccurate bills as a result of an error that arose from June 2016.
The regulator had hoped to complete its "evidence gathering" by November 2018.
However, Ofcom has said it has "reasonable grounds" to believe Giffgaff provided "inaccurate information in response to two information requests sent during the course of our investigation."
Back in June last year, Giffgaff promised to refund customers after an error meant data would continue to drain their credit rather than be extracted from the allotted goodiebag allowance.
A Giffgaff spokeswoman said: "As this is part of an ongoing confidential Ofcom investigation, at this stage we're unable to provide any further information."
The regulator has also opened a separate probe into O2, the network on which Giffgaff is based, where it said the operator had experienced "extraordinary performance failure" of a billing accuracy issue.
It said: "That notification also suggests that from at least 1 January 2012 – 7 March 2019, O2 took duplicate final direct debit payments from customers who terminated their contracts on a Saturday or Sunday and had an outstanding periodic bill to pay."
Meanwhile, EE and Sky both face an investigation into possible breaches of net neutrality. ®