Samsung Galaxy S2 — network sim unlock unsuccessful

I own an s2 and an s4 from tmobile. In my area tmobile reception is really bad, so I asked tmobile for sim unlock codes and they gave me both codes. The code for my s4 worked with no problems. The s2 didn't. I got the message network sim unlock unsuccessful. I know the code is accurate because I also took it to a place that unlocks phones and they received the exact same unlock code. The technician told me that he has had a few tmobile s2's come through that gave him the same problem, but the majority of that particular model he has never had a problem unlocking. As you know this was an expensive phone and right now its more like a paper weight. Can you please help me with this. I moved my s4 to at&t and I want to do the same with my s2.