As far as I’m aware when you register for my Three for the first time,the system should send you a one time four digit passcode to your phone to enter as part of the registration process. You don’t say whether you’re a payg or contract customer,the reason I say that is because if you’re having issues and you’re a contract customer you can contact customer service on 333 from your handset. If you’re a payg customer your only way of contacting them is via live chat on the Three website.
I sincerely hope this is of some help to you.
