Can thieves still use the iPhone?

I have two questions, 1. If an iPhone has a lock on it can you still use it even if you don't know the code and 2. If the phone is frozen because it's been reported stolen can it still be used at all

If they are pretty good hackers they can make it enter dfu mode and hadd restore it but it also depends what carrier you have if you have verizon or sprint they can't use it unless it is an iphone 4s where they can unlock it. And use it on att kr tmobile or any gsm carrier for that matter but if it is a iphone 4 or.lower theh can't use it as a phone if it is verizon or.sprint because the employee should notice it was on another persons account unless they don't pay attion or the.people are again good hackers but they can always just use it kinda as an ipod touch so yeah they can use it but maybe not as a phone