If their Number is in a contact list

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Over-Permissive Apps: Many apps request access to your contacts, sometimes without a clear necessity for their core function. Granting this access means the app can read, and potentially upload, your entire contact list to its own servers.

Third-Party Data Use: Once an app has your contacts, its own privacy policy dictates how it uses and potentially shares that data. This is a major concern with social media and messaging apps that build "social graphs" based turkey phone number list on contact lists.

Shadow Profiles: Even if someone isn't on a particular social media platform,uploaded by a friend, a "shadow profile" might be created for them.

Crowdsourced Databases (e.g., Truecaller): Apps like Truecaller rely on users uploading their contacts to build a vast database of phone numbers and associated names. If your number is in someone else's contact list and they use such an app, your number could become publicly identifiable even if you never signed up.