When you enable Contact Sync, Telegram securely uploads the phone numbers and the names you've assigned to those numbers in your device's local address book.
Limited Data: Telegram explicitly states in its Privacy Policy that it "only need the number and name (first and last) for this to work and store no other data about your contacts." It does not access or store other details like emails, physical addresses, notes, or company names from your device's contacts.
Secure Upload: The data (phone numbers and names) is uploaded to Telegram's cloud servers using its proprietary MTProto protocol, ensuring it's encrypted during transit.
Backend Matching: Telegram's servers then perform telegram number database a rapid matching process. They compare the uploaded phone numbers against their vast database of registered Telegram users.
Identification and Association: For every phone number from your device that matches an active Telegram account, Telegram identifies the corresponding Telegram User ID. This allows Telegram to link your saved contact (e.g., "Mom") to their Telegram profile.
Cloud Storage: The identified contacts are then added to your personal Telegram cloud contact list. This list is associated with your Telegram account, not just your device.
No Raw Address Book Storage: Telegram does not store a raw copy of your entire phone's address book on its servers. It stores the relevant data for contact matching and display.
4. Cross-Device Behavior:
Since your Telegram contact list is stored in the cloud (associated with your account), it seamlessly syncs across all your logged-in Telegram devices. If you add a new contact on your phone and sync is on, that contact will also appear in your Telegram app on your tablet or desktop.
5. Privacy Implications and Control:
It's Opt-In: Contact Sync is always an opt-in feature. Telegram asks for your explicit permission before accessing your device's contacts. If you decline, you can still use Telegram, but you'll have to add contacts manually.