Mobile phone giant Vodafone has made hundreds of changes to its computer systems following a flood of complaints about bills.
Figures from regulator Ofcom showed complaints about the operator hit three times the industry average by the end of last year.
Vodafone also faces the prospect of a heavy fine from Ofcom for the way it handled complaints.
It has now fixed 350 elements of its IT system to reduce problems.
The company said that some were “tiny tweaks” but others were correcting issues which had a “significant” impact on customers.
Vodafone had moved all its billing and other services onto one system, alongside a change to its customer contact centre.
“The combination of new systems, new processes and new customer service agents impacted service levels significantly,” a spokesman for Vodafone told BBC Radio 4’s Money Box.
That impact led to complaints to the regulator reaching 32 per 100,000 pay-monthly customers, compared with an industry average of 10, in the final three months of 2015.
These figures underestimated the true levels of complaints, as not all customers with problems would have alerted the regulator after speaking to the company.
Vodafone is also preparing a response to investigation findings by Ofcom which concluded that there was reasonable grounds to believe that the company did not have the correct procedures in place to deal with complaints over a two-year period. The company could be fined up to 10% of its turnover.
Ways to complain
Among those unhappy with Vodafone was Hayley Angell, who lives in Scotland.
The 31-year-old wanted confirmation that her phone had been blocked after it was stolen.
She said she made repeated calls to the company’s contact centre, but only had success after writing letters to the entire Vodafone executive. She was released from her contract and switched to another provider.
Other cases include that of Fiona Campbell, a photographer from London, whose details were mixed up with a customer of the same name who lived hundreds of miles away.
She said the turning point for her complaint, after four months of deadlock was when she posted a message on Twitter threatening to leave.
Solicitor Peter Felton, from Kent, said he complained twice after paper bills, which he was happy to pay for, failed to be delivered.
“I feel that talking to Vodafone customer service means I may as well just talk to my coffee cup,” he said.
A Vodafone spokesman said: “It is clear that each of the customers you have brought to our attention have been let down by errors caused by customer service agents not sticking to the processes we have in place to correctly identify and resolve issues as quickly as possible.
“We take every case extremely seriously and aim to fix them as soon as possible without any financial impact to customers.
“We are always disappointed when customers needed to raise complaints with us and, more so, when we then do not resolve them effectively first time.
“Our teams, at all levels, want to provide our customers with a great service every time but we are clearly not there yet, so are investing significant resources to do so as quickly as we can.”
So if you have a problem with your mobile company, what is the best way to complain?
Regulator Ofcom says there is a specific timeline to follow when making a complaint about any telecoms provider.
“Consumers who are experiencing problems with their communications service should firstly raise the issue with their provider and follow its formal complaints process,” a spokeswoman said.
“If their complaint remains unresolved after eight weeks, or sooner if ‘deadlock’ is reached, consumers have the right to escalate it, free of charge, to an Ofcom-approved dispute resolution scheme. The scheme will assess the case and reach a fair and impartial judgement.”