Pay online with a mobile phone?

So, I would like to buy an item in the Xbox store using a mobile phone, but now I don't know how it works properly. So my questions are:

Do I need credit on my mobile phone to pay? (Have a contract, not prepaid.

If not, will the amount simply be added to the contract costs next month?

And if neither applies, a brief explanation of how the payment works would be very nice.

I thank you in advance for any help.

So what do I do via PayPal

If I were, I would have PayPal. I'm only 15, so not an option for me.

Then you have to talk to your parents:-)