iPhone 6 come unlocked until sim inserted then locked?

[Anonymous]

Just wondered if O2 Pay Monthly iPhone 6's come unlocked, and only lock to the first sim which is inserted? 

I ask as I currently have an iPhone 6 which I got from Three Pay Monthly (Assuming therefore it is unlocked), however I would like to get the 6 Plus, my sister therefore has an upgrade with O2 at the moment, so I can give her my 6, and take her 6 Plus....Is this viable?

Will be bought from O2 online / Phone. So if I put my three sim inside when it arrives it'll accept it? And her O2 sim will work in my three (unlocked) iPhone 6, which is confirmed to work, tested tonight. 

Thanks

[Anonymous]

Whilst i agree with what everyone is saying, it's worth noting that if you buy it from the Apple Store, the only way to make sure it is fully unlocked is to buy it SIM free. If you buy it on contract from the Apple Store, it will lock to that network when they activate (trust me, I've been through a nightmare dealing with this in the Apple Store!)