Hi @Blinkonce. As you mentioned you kept your old number, the info shown will be the original number assigned to the SIM (despite you keeping your old number).
So, did that work for you? I tried to do that a couple of times on different contacts, and it just doesn't work. I mean, as soon as I press on save, it tells me everything is fine, but when I check it, nothing changes. I even went to the local store to see if maybe there was a certain problem with this phone, and they also couldn't understand why that was happening. My s20 ultra is probably the worst phone I have ever had. I cannot even receive sms online australia, though I am connected to good wifi with a stable speed and so forth.
@worthingtonducote: The number shown in Settings > About Phone is automatically generated by the registered SIM card, and cannot be manually changed. On occasion, a read error can cause the wrong number to show here, or it will say 'Unknown' where the number should be populated. I understand that this isn't ideal, however please be assured that it will not affect your network capabilities in any way.