I have a totally unknown number shown for my device in the "phone info" section.

[danjoy42]

how is it possible and how can I change it?

[WelshPaul]

You can't change it on a SAMSUNG phone but you can on an Oppo.

[Cliffg]

The phone number is actually stored on the SIM. When I wanted to add my number I had to put the Sim in a friend's IPhone. I could then edit the number. Putting it back in my Samsung it read the number OK.

[AndrewL]

@danjoy42: SIM cards contain a temporary placeholder number before your own number is added by your mobile network provider. On occasion, this can cause a read error in Settings > About Phone, causing it to show as 'Unknown' or, in your case, to show the default number. Typically this doesn't affect your network capabilities in any way, however you may wish to contact your network to request a SIM swap to see if this resolves the issue. 

[Members_C5v8tuI]

Had to break my head to fix this issue, but eventually found a solution! Honest opinion- android/samsung suck with their sim networks. Take your sim and just put it in an iPhone, wait for a few seconds till the phone reads the sim. Then check the settings on the iPhone if that's the right number you're looking for (you can also edit the number in the settings). Then take the sim out of the iPhone and put it in the samsung and you should be good to go. Worked like a charm for me.