Chow.X.Q doesnot what he is talking about. ignore.
even though you debranded your phone, is still remains locked to network if it was locked at the time of debranding.
in your case i assume it is locked to orange, and if this is the case then only simcard you can use is from orange.
depending on your location, i know that some shops in UK "patch unlock" your phone, what this means is that phone is not completely unlocked so if you flash your phone with firmware after the unlock it will lock up to network again.
T-mobile in UK will provide you with unlocking key if you are PAYG customer, as for orange i dont know.