Format of phone numbers

I am in the UK, where phone numbers have the format 01234 567890. My iPhone SE keeps re-setting this format to 01 234 56 78 90. In this format, when I try to make a call, I get a message saying I don’t have enough credit to make this call - which is nonsense. If I use the keypad to enter the same number manually, it works fine.

I have tried turning off Dial Assist, but that doesn’t help.

If I change my regional settings to some other location (e.g. US) and then back again to UK, the format and calls are corrected - but only temporarily. Within a short time, the format reverts to 01 234 56 78 90. Does anyone else have the same problem? Is there a solution?