International phone number format on iPhone 11 in the UK

Hello, I'm based in the UK and have my contacts in my iPhone 11 set up with phone numbers prefixed with +44 (0).. followed by the phone number with the initial 0 dropped. All these are UK based contacts. So for example

01234 567890 shows as +44 (0) 1234 567890

I use one of these contacts regularly and they had an iPhone 5S. I had no problems using the contact for this person. All calls were successfully connected.

Today this person moved to an iPhone SE. They moved service provider and migrated their date/settings. But when I tried using my contact card my call seemed to connect and just rang out. Indeed at the other end the recipient heard my call come in but was unable to answer. It just rang out until an automated voice told me the call could not be connected.

On my iPhone 11 I decided to simply call the UK (rather than international). I.e., the 01234 567890 number and I got through. The person answered the call successfully. So then I changed the contact card accordingly and again I successfully called this person using the contact card.

After a little research I found that the international dial +44 1234 567890 (the 0 has been removed totally) is an acceptable format. So again I updated the contact card and used it to call the person and the call was successful.

I don't know what the problem is here or indeed where the source of the problem lies; is it the SE or 11?

On my iPhone 11 the phone number field in a contact card seems to accept local dial, +44 and +44 (0). I looked at the language & region setting on this phone and I think it was set to English (US) so I changed it to English (UK). But I'm not sure if that makes any different apart from the spacing between digits in the contact phone number.

Note: I migrated from a 5S to 11 back in September but didn't experience any problems with contacts and calls.

Any one got any experience/advice around this

Thank you,

Andy