Welcome to Vodafone Community
I'm trying to use the form at https://vodafoneinlifeporting.co.uk/ilp/web-forms/pay-as-you-go-to-pay-monthly/,
but it needs a 20 digit SIM number. My SIM card only has 19 characters, and the form won't submit without the full 20. How do I transfer my number over?
That's not good as it should be a simple process as set out in this link > my-vodafone-account/keep-my-number. which leads to the form you've linked to !
In your phone settings it may also show in the About Phone Settings
If not then Customer Support on 191 or Live Chat can help.
Current Phone > Samsung Note 20 5G - SM9860 Snapdragon.
Samsung One Ui 2.5 / Android 10.