Add another payment method first, then remove the mobile phone billing.
See -> Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)
See also -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card.
If your card is not accepted as a paymenf method, you need to contact your bank.
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 -> Add money to your Apple Account balance
Add money to your Apple Account balance - Apple Support
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 or check -> here for more support and assistance.