Facebook (FB) has been hiring outside contractors to listen to voice calls on the Messenger App and transcribe them, Bloomberg reported, raising more concerns about the company's record on user privacy.

Users whose messages had been transcribed had chosen to do so, but employees transcribing these messages had no idea how the audio was obtained or where they had been recorded.

Facebook said the transcriptions were used to improve its artificial intelligence technology, and said the practice was stopped a week ago.

Facebook has been criticized in the past for its policies on user privacy. Last year, the data polling firm Cambridge Analytica, which was hired by the Trump campaign during the presidential elections in 2016, was found to have used Facebook to gather information on the profiles of millions of Americans. Facebook CEO Mark Zuckerberg was asked to come to congress to testify on the issue. 

The company has had to pay $5 billion to the Federal Trade Commission (FTC) due to privacy violations. 

Voice services from other tech giants have also come under scrutiny. Amazon's Alexa voice assistant raised controversy in several U.S. states after it allegedly collected recordings of children without their consent. 

Apple contractors have regularly heard confidential information on recordings obtained by the Siri voice assistant. Apple says some Siri requests are analyzed by outside contractors to improve the service and its dictation capabilities. 

Facebook's Messenger app, part of the Facebook social network, was released as a standalone app in 2011. As of November 2018, the app has more than 1.3 billion users