You can prevent users from contacting you if they have your phone number and you don't have their phone number: There should be a let my contacts know i'm on telegram option when you make an account. If you can afford it, use another number to sign up and then change your phone number. Secret chat uses telegrams own mtproto protocol which has been criticized by cryptographers in the past. Telegram has improved a lot there but they don't know what they're doing really.
Telegram think that people being given your telephone number is a.
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