About seeing your phone number already in WhatsApp
All WhatsApp accounts are tied to mobile phone numbers. Since it's common for phone numbers to be recycled by mobile providers, it's possible that the previous owner of your current phone number used WhatsApp.
If the previous owner of your phone number didn't delete their WhatsApp account, you and your contacts may see your phone number in WhatsApp before you activate a new account. You may also see someone else's profile photo and About section attached to your phone number.
There's no need to worry. This only means that the previous account wasn't deleted, so old information still exists in the system. This does not mean that the previous owner of the phone number has any access to the WhatsApp account you activate with your new phone number. Your conversations and other WhatsApp data are secure.
To help eliminate confusion with recycled phone numbers, we monitor account inactivity. If an account is unused for 45 days and then becomes newly activated on a different mobile device, we take this as a sign that a number has been recycled. At this time, we'll remove the old account data tied to the phone number - like the profile photo and About section.