25 November 2016 Last updated at 16:04 GMT
A report from Hong Kong investigative news agency Factwire found that apps CM Security, Sync.ME and Truecaller have built up databases with billions of people's phone numbers - including top politicians in Hong Kong and the UK.
The apps let users "reverse look-up" calls from numbers they do not recognise - but take much of their data from users' contact lists.
CM Security says it has temporarily halted its "reverse look-up" function to address privacy concerns, while Sync.ME and Truecaller say people can opt out from their databases via their websites.
Read more: http://www.bbc.com/news/technology-38103895
Reporting by the BBC's Helier Cheung in Hong Kong
Three mobile phone apps that help users block spam calls have been building up searchable databases of people's names and phone numbers - sometimes without their permission.