Xbox live Gold payment due?

I currently have 20 euro in debt and would like to pay it off, but can only pay by credit card, PayPal or over the phone. I also have enough money in my account but can't pay with it. My question is how can I add this payment method?

Not at all and you describe it yourself!

"can only pay with credit card, PayPal or a phone call"

Payment with credit is no longer possible in your case because you have already been unable to make a payment due to insufficient funds. With the alternative payment by credit card or PayPal, Microsoft wants to ensure that something like this does not happen again.

A little tip for life that also applies to all other regular payments such as subscriptions, telephone, TV, Internet, rent, electricity, insurance or other payments.

Only do this if you can ensure that you can follow the payments.