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Some Vodafone customers have been overcharged thousands of dollars but the telco is tight-lipped about its "revolutionary billing system".
Vodafone denied Herald on Sunday requests for an interview and refused to answer certain written questions, instead issuing a brief statement through spokeswoman Alison Sykora.
The company admitted having problems with some business customers on the talkZone plan, which allows employees from the same company to talk to each other on their mobile phones at no additional airtime cost.
The Herald On Sunday understands customers have been billed for what should have been free airtime.
One customer said his company had been overcharged since the end of last year when he pointed out the error. His general manager described Vodafone's billing system as "a disaster".
Vodafone had agreed the bills were incorrect but the overcharging, which had topped $20,000, was still occurring last month.
The Herald on Sunday understands a number of fulltime staff are to sort out the overcharged accounts.
Vodafone would say only that it had put "a huge amount of resource" into rectifying the problems.
It said talkZone problems affected only 150 business customers and most issues had been resolved.
The rest should be cleared within three weeks.
They would not say if non-talkZone customers were affected or if any other people had been overcharged.
Nor would they say why it had taken at least seven months to rectify the overcharging problem if only 150 customers were involved.
