Major mobile phone service provider Vodafone K.K. said Wednesday it made 13 million yen worth of billing mistakes in a total of 624 cases between 1998 and 2005.
The cases involve both overbilling and underbilling, according to the company.
Vodafone will reimburse the customers it overbilled but will not ask for payments from those it underbilled, it said.
The overbilling mistakes came about because the company failed to provide eligible customers with the family plan, under which it gives discounts if more than one family member uses its services.
The amount of money and the number of cases could increase as the carrier tabulates data for people who have canceled their contracts, Vodafone said.