Three Ireland is to refund over €2.6 million to customers after charging them for cancelled services on their contracts.
Telecom regulator, Comreg, examined the company's billing system and found that Three had been keeping post cancellation charges as credits on inactive accounts instead of refunding them.
It found Three were only granting customers the refunds if they requested them.
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The review also discovered that other forms of credits not related to post-cancellation charges were held on inactive accounts.
These charges had not been claimed or proactively refunded by Three after contracts were cancelled by customers.
In total, 173,000 customers were impacted by the issues with €1.28m in post-cancellation charges and €1.4m for other credits owed.
Three are to begin the process of refunding their present and past customers.
The process is expected to begin this month and is estimated to take six months to complete.
The money will be refunded through electronic fund transfer to the customers' accounts.
The company has said future credits will be proactively refunded and will not be left on inactive accounts.
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