Customer complaints about Vodafone soared towards the end of last year, according to the latest report from UK telecoms watchdog Ofcom.
The regulator says it recorded 32 complaints for every 100,000 of the firm's pay-monthly subscribers over the last three months of the year.
That is more than double the figure for the same period in 2014, at which point Vodafone was still the most complained about network.
The company blames an IT glitch.
The actual number of frustrated customers was probably higher as Ofcom only tracks the number of people who contacted it directly and not those who only complained to Vodafone itself.
"The points highlighted in this report largely relate to a major programme we undertook last year to transfer customers on to a new billing system, aimed at simplifying the operation of their accounts and opening up a range of better services, such as 'click and collect'," said a spokeswoman.
"Unfortunately, there were some problems during the highly complex migration. Now that the migration exercise is essentially complete, we expect our £2bn investment in our network and services will start to deliver a step change in customer experience."
Vodafone said it had about six million consumer pay-monthly subscribers out of a total base of 18 million UK customers.
Talk Mobile was the only other mobile operator to experience a rise in pay-monthly customer complaints over the three months - but at nine complaints per 100,000 subscribers, its rate was much lower.
Tesco Mobile generated the lowest relative volume of complaints at just one recorded grievance per 100,000 customers.