[cybersh]

Once whatsapp is transferred by iTunes from an old to a new iPhone, is it fully working? or do we need to re-login? (with my phone number)

[Tartarus]

You will need to re-login, unless you chose for an encrypted backup.

[cybersh]

Thanks Tatarus,
Just to confirm I got your point : By "encrypted" you mean I have to ask iTunes to encrypt my iPhone datas?
... Or what?
(see picture)

[Tartarus]

Yes, that's exactly what I mean.

[cybersh]

Thanks again !!
Well ... Otherwise, if my backup is not encrypted, I could still use my whatsapp account with the old phone?
(Even if my backup is transferred to a new iPhone, assuming I don't re-login to whatsapp with the new one)

[Tartarus]

You can use WhatsApp on any phone you'd like. You only have to be able to enter the passcode sent to the number you've entered to use it with. So as long as you have control of that number and are able to see that sent code, you can use WhatsApp on any phone, but not simultaneously. You'll get logged out once you set it up on another phone.