Not prompted to enter device unlock code when entering different sim card

What sim card are you trying?

I've tried Cellcom, Sprint and Verizon.

Don't know about cellcom but Verizon and Sprint won't work with the att version of the key

The phone won't work on sprint or Verizon, but should trigger the pop up to enter code. 

That's so strange...what's the point of AT&T providing an unlock code if it can't be unlocked? I see versions of this phone that have been "unlocked". Like I said...strange.

Just for fun I just went out and tried US Cellular and T-Mobile, neither triggered a request for unlock code or worked. The Cricket card I had put in worked fine...as it should being an AT&T company. 

Neither of us said it can't be unlocked.  But it would work on T-Mobile or its mvnos. 

Cell Com is a Verizon partner, so it won't work on them. 

I read the thread again, in your first post you said you got cellular network not available after factory resetting the phone. The usual notification is Sim not compatible.

Since you have tried it with T-Mobile and did not get a pop up to unlock the phone did you also get cellular network not available or Sim not compatible? Did you use an active T-Mobile SIM card?

I'm thinking along two lines, either the phone cannot be unlocked just like the fire phone, razor 2, and red hydrogen one, which means they would not have even given you an unlock code, which makes me dismiss that entirely.  

Or than the phone is not locked.  Wild thought, but if it worked on T-Mobile that might confirm.  Are you the original owner? Might it previously been unlocked? 

I'm not the original owner. Since that one time, I haven't received the cellular network not available error since. The T-Mobile card I used was not active  but that hasn't been a problem in the past. If the network isn't compatible, working or not, the phones have rejected the SIM or asked for an unlock code.