I have done a fresh install , i chose i dont have a key and it got activated automatically like you said but i am confused about the keys, I have a windows 8 key and a media center key , when i checked the key through magic jellybean key finder it showed another key , so which would i have to use in case i have to get it activated on phone if needed ?
you should never need to do phone activation but basically what happens is this. You upgrade your windows 8.1 machine and as long as it's a Genuine copy as well as activated your issued what's called a 'Digital entitlement' . This is your Windows 10 licence and is stored online.
The Windows 10 licence is OEM meaning it's tied to the machine you upgraded on. Even if your Windows 8.1 licence was retail Windows 10 will still remain an OEM copy.
As I said you should never really find yourself in the situation of having to phone activate no matter how many times you clean install.