As far as I'm aware, sim free means it will work with any network. The only restrictions I can imagine would be the area coverage by x network, but you could check area coverage with a google search I'm sure. Hope this helped! Sorry if it didn't
'Sim free' means exactly what it says on the tin, it'll work on any mobile network (once you've an active micro-sim card). I bought this sim-free 820 recently and used my O2 micro-sim card in it without any issues, my brother bought a sim-free 920 a while ago and it works fine with his e-Mobile micro-sim card.
As far as I know, if the network can provide you with a sim then it will work; I don't believe there are any restrictions. But to be certain pop into a provider and check before you buy; just tell then you already have a phone. All the providers in my area offer pay as you go, or talk only contracts.