Dear @zaki2000 ,
welcome to our community.
You need a German address and a German bank account for a cell phone purchase. After the purchase of a cell phone, there will be a credit check, which you have to pass in order to conclude the contract. Sometimes an increased deposit is then required via a so-called accepotmail. This increased down payment can only be made via the sent email link and the down payment will then be offset against the following installments until it is used up, after which your account will be debited monthly.
It may be that you will have to show your ID card when you buy in an o2 store because the ID cardnumber is required. As far as I know this is not the case with online purchases.
Does this help you?
Kind regards,
Andrea