My iPhone says I need to do billing
There is a good chance you have an unpaid balance from a previous transaction. This can happen for a number of reasons such as insufficient funds in your bank account.

If you have an unpaid transaction, iTunes requires you to add a valid payment method before you can continue using the iTunes Store/App Store. Even for free items.

You can see if you have any unpaid purchases by going into your 'Purchase History' (see example below) or contacting iTunes Support via email or phone.



More info: https://support.apple.com/en-au/HT203005