If you brought from Apple itself, the chances are it might not be locked to one network then. It is only when you buy from a phone store that they are locked.
However, some phones are made to work with one network only, so you have to jail break the phone.
I used to have a 3g iphone about ten odd years ago, and I believe at the time, you could only get them from one network provider. The same happens in the states as well where AT&T are the carriers.
I could be wrong, but if you have your old sim that used to work, insert that and see if you get a network signal, it should say what carrier it is.
E.G I was on O2 when I purchased my phone, I then transferred onto GifGaff which is basically the same company, so the phone sim would work.
Regards
Mark
