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.
Thanks for posting. Please can you do the following;
Settings, mobile data, SIM applications, roaming, select mode. Manual. Then chose national.
Make sure your numbers are set to the correct format. If not change region to anything other than UK (settings, general, language and region, region) and then change back to the UK.
Need a helpful hand to show you how to make a payment? Check out our guide - How to pay my Virgin Media bill