iMessage linked to 2 numbers

Hi guys, can anyone help me with this. I was looking through my iMessage settings. And I noticed that this setting “you can receive iMessage to and reply from” and below 2 phone numbers are “checked”. My number and an unknown number.

At first I though it was a number created by Apple to link to iMessage. But I googled and found no one else having this issue.

Can anyone advise me if your phone is the same? Or has my phone been hacked or something?

thanks in advance.