Data privacy watchdog the Information Commissioners Office has featured the scam in its proposal to government arguing for jail terms in cases of serious Data Protection Act breaches.
Christopher Graham, the Information Commissioner, said yesterday that a mobile phone company had contacted his office warning of the scam - but did not name the firm.
However T-Mobile has since revealed that it was one of its employees, no longer with the company, who sold thousands of records to brokers.
The middlemen would then attempt to sell the data to rivals in order to contact owners before their contracts expired and offer them a new deal.
T-Mobile, which has 16.6m UK customers, says it has "since put systems in place to minimise the risk of it happening again".
Details of the breach were contained in a submission by the ICO to the Ministry of Justice seeking jail terms for serious data protection infringements.
In its submission the ICO said that a prosecution case "is now being prepared".
It confirmed that the number of records involved "runs into the millions" and that "substantial amounts of money changed hands".
Graham, who took over the IC role in June advocating zero tolerance of data privacy breaches, is supporting government proposals for people who steal and sell on personal data to be jailed under Section 55 of the Data Protection Act.
The current maximum punishment is a fine.
"If public trust and confidence in the proper handling of personal information, whether by government or by others, is to be maintained effective sanctions are essential," Graham said.
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