Locked iphone 4
I have never owned an Apple product before but have now been given an Iphone 4 to try out. The phone was originally purchased from the Apple store and connected to the Vodafone network. My father later switched to O2 in 2011 and has been using it pay as you go since then. I now wish to use it on a different network but find that the phone is now locked to O2.
Is it the case that using the O2 sim has locked the phone? I would have understood if the phone had locked to vodafone as this was the first sim ever used in this particular phone. I am thinking that maybe the locking process was carried out during an ios update when the O2 sim was in use. Would this be the case?
I am just a little confused as to why the phone is locked now but not when it was purchased and that I have to now purchase an O2 sim and load it with £15 credit before they will unlock it.
iPhone 4, iOS 7.1.2