If I send someone a text, how much information am I giving up? Could they add me to an app like Whatsapp and access my name and profile? Or does their number first have to be saved in my phone contacts for that to happen? What other information could they access just from a text message, assuming I have not added them as a contact?

  • It is possible to link to a Telegram with their phone number. For example, if you know someone's Telegram, then you can create a contact on your phone and guess their number by repeatedly changing it until it shows up on their Telegram profile (even if they have the strictest settings), meaning you found their number. So someone could probably do something similar to find a Telegram profile starting from a cell number and guessing Telegram accounts by adding them. The process could maybe be automated to go faster.
    – mbomb007
    Jul 21, 2020 at 20:45
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    anyway, this breaks down to "If I give someone my phone number, what can they do with it?" And the answer depends 100% on where you used that phone number as identification. Or whether you'll be annoyed if you get called in the dead middle of the night. Jul 21, 2020 at 21:04
  • @MarcusMüller how does it depend on where I used the phone number? Let's say I text someone randomly - can they find out my name, profiles, etc?
    – kandyman
    Jul 21, 2020 at 22:25
  • @kandyman it depends on where you used the number; e.g. what accounts it is associated with and whether each of those services allow you to search by phone number or the like. E.g. I believe Facebook lets you find someone if you have their phone number and the person who added the number has not disabled this feature. Jul 21, 2020 at 23:10
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    If anyone adds your number as a contact AND your number is your Whatsapp contact, the name you choose for yourself and your picture are visible to them. And your last online time too.
    – ThoriumBR
    Jul 22, 2020 at 1:12

2 Answers 2

  • If you publish you mobile number in a directory such as Spokeo they can look you up

  • If you use Telegram, the other person will be able to see your public name. Your last seen online time could be retrieved either precisely or imprecisely depending on your privacy settings - this information cannot be hidden completely. Telegram will always tell anyone whether you're currently online (using the app) or not. You can make your profile photo visible only for your contacts.

  • If you use WhatsApp you can hide your last seen info and profile photo completely from everyone. Your public name can also be seen by anyone regardless.

  • It's trivial to find out what your cellular operator is and in many cases it's not difficult to pinpoint the location where the SIM card was sold to you.

  • With WhatsApp, how is the name seen? Does it automatically add an unknown number and retrieve profile info?
    – kandyman
    Jul 22, 2020 at 7:52
  • WhatsApp retrieves the info as soon as you add another person to your contact list. Jul 22, 2020 at 8:16
  • In my question I said “assuming I have not added them as a contact”, but do you mean that knowing my number allows someone to add me as a contact, thereby allows my them to see my WhatsApp name and profile? Even if I haven’t added them?
    – kandyman
    Jul 22, 2020 at 9:00
  • that knowing my number allows someone to add me as a contact, thereby allows my them to see my WhatsApp name and profile? Even if I haven’t added them? Correct. Jul 22, 2020 at 9:04
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    At least in EU we can keep our phone number when switching cellular operators, so it's not possible to guess the operator from just the number.
    – Marki555
    Aug 15, 2020 at 12:54

To answer the question yourself I suggest you to look around for some old SDKs like Siemens included to its cellular phones (S50?) with some tools i.e. something like SMS Builder to test your apps.

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