I've had this issue since January 2020, now over 2 years on and having been on to Apple support over 20 times, tired everything under the sun I am convinced it is an issue on Virgin Mobile's side.
I have just read through this entire thread and tried all the 'fixes' including the one above and still I have the issue.
THE NAIL IN THE COFFIN FOR VIRGIN MOBILE:
On icloud.com my contacts all show in UK format.
On my iPhone > Settings > Contacts > Accounts > iCloud > Turned off my contact, restarted the phone and turned them back on from icloud... They are still in Belgium format.
Swapped out my SIM for a new EE one and follow the same process.. Hey Presto the contacts now show in UK format.
Annoyingly I'm still in contract until Aug 2023 but I am now going to start the process of cancelling my contract with them on the grounds they aren't providing the service I am paying for.
Massively disappointed that a company this big can't solve a seemingly simple problem. My advice to anyone reading this is to cancel your contact or once it's up move to a different provider.
