1. Re-install whatsapp on the same phone and same SIM : you have to go through the entire setup process again. You're asked for your phone number, whatsapp sends a sms to that number to confirm. If you had previous backup of chats, it restores that.
2. Insert SIM into a new phone, without whatsapp previously installed: Same as 1.
3. Change SIM in the same phone and whatsapp previously installed: Nothing changes. Whatsapp runs using your previous number and that is displayed to all the contacts. On re-installing, 1. is repeated.
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How does WhatsApp deal when you change your SIM card into a new phone? I mean, how does it know it's you on your new phone?
I have tried some of the cases:
1. Re-install whatsapp on the same phone and same SIM : you have to go through the entire setup process again. You're asked for your phone number, whatsapp sends a sms to that number to confirm. If you had previous backup of chats, it restores that.
2. Insert SIM into a new phone, without whatsapp previously installed: Same as 1.
3. Change SIM in the same phone and whatsapp previously installed: Nothing changes. Whatsapp runs using your previous number and that is displayed to all the contacts. On re-installing, 1. is repeated.
When you install you make a new password by sending an SMS to your phone to verify that you actually own the number.
If you change a SIM card in a phone it will keep using the same Whatsapp number
I like the use of the word "password". If they would at least implement a password, the whole thing would be a lot more secure.
They should have made the password random, it's saved anyways.