If you do not mind porting out, that would be the easiest way. A transfer of ownership can be done as well. https://www.koodomobile.com/en/help/transfer-of-ownership-town

I tried to do a TOWN but it’s not possible without the verbal agreement of my father, and I haven’t looked into Porting out yet, I would prefer to take ownership of the account, but haven’t been successful with that. 

Only account owners have the right to transfer the number. The restriction was there to protect customer. I recommend you communicate with your father first. If that doesn’t work out, you might just go ahead and get a different number on your own.

You can try to port out and see if you are able to port the number with the account info.

Otherwise, you will need to speak with your father to complete a TOWN.  There is no way around this.