TalkTalk has emailed its customers to warn that hackers have managed to steal account numbers and names from the company's database.
The email states that scammers are using the stolen data to trick customers in to handing over bank details.
The theft was uncovered following a sudden rise in complaints about scam calls between October and December 2014.
"We have concluded a thorough investigation working with an external security company, and we have become aware that some limited non-sensitive information may have been illegally accessed in violation of our security procedure," confirmed a TalkTalk spokesperson.
"We are aware of a small, but nonetheless significant, number of customers who have been directly targeted by these criminals and we have been supporting them directly."
Information stolen included names, addresses, phone numbers and account numbers, the telecoms operator added it was "confident" no sensitive or payment data was taken.
TalkTalk was targeted through a third-party which also had access to its network, legal action has is now being taken.
A dedicated phone line has been set up for customers affected.