How do I add mobile billing as my payment method. ?

1.

You need to contact your carrier, see this Apple article: If there's an issue when you set up mobile phone billing

If there's an issue when you set up mobile phone billing - Apple Support (MD)

• Mobile billing is currently available only in some countries and regions. 

2.

I don’t have a credit card.

If you don't have a credit card or don't want to use a credit card, you can also use your Apple Account balance as payment method.

See -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

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Where to buy a physical Apple Gift Card or App Store & iTunes gift card: at an Apple Store or an Apple Authorized Reseller in your region.

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes Gift Card – Apple Support (UK) 

When redeeming a gift card, the amount will be credited to your Apple Account balance and you need to set your Apple ID balance as your default payment.

See also:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support