BT surrended information on the partner of a man the investigators were tracking.
Christopher Hackett, from Swift Investigations, posed as a BT employee. He phoned the telecoms company's call centre and deceived a member of staff into revealing personal information on the man's partner. That information allowed Hackett's client, Blackhorse Finance, to visit her home.
The man was wanted by Blackhorse for an outstanding debt.
Pleading guilty to unlawfully obtaining and selling personal information, Hackett was fined £400, while a middle man, Darren Whalley, was fined £500. Both were ordered to pay £400 towards prosecution costs.
Following the incident, BT contacted the Information Commissioner's Office, who commenced the prosecution.
Though the ICO says that BT was wrong in disclosing the information, a spokesperson confirmed that it will take no action against the telco.
A BT spokesperson described the incident as an "extremely rare occurrence," and he said no changes were being made to company procedures. "We have systems in place to pick up this deception," he said.
Mike Gorrill, assistant commissioner at the ICO said that illegally obtaining and selling personal information is a "serious offence".
He added: "This prosecution ... is part of our ongoing work to stop the illegal trade in personal information. Individuals must be confident that their personal information is stored securely."