Cannot update payment information. Contact Apple Support for more information

Hi - hoping someone can help with updating payment method on iphone as I seem to be going around in circles =(( Thanks in advance!

When i try to add a new payment method i get the following error message: 'Cannot update payment information. Contact Apple Support for more information'

Actions taken:

• Message notification received: 'Update Payment Details. There was a problem with your iCloud storage payment. Update your payment details to keep your 2 TB of iCould storage. [existing credit card has expired]
• I clicked on payment and 'Add Payment Method',
• Entered details of new credit card
• Error message: 'Cannot update payment information. Contact Apple Support for more information'

Things i tried:

• Different credit cards
• Changing payment details on https://appleid.apple.com, but it comes up with an error on the phone number field (don't know why since the number on record hasn't changed).

Background:

• Iphone 8
• Software is up to date. iOS 13.3