dumb question about unlocking

Ok so here is a dumb question for you. when does one get a phone unlocked? Can an inactive phone be unlocked? or does it have to be active? sorry, older and not really that knowledgeable.

 

I have a MetroPCS Samsung Galaxy Exhibit I used for a year and a half (bought new from MetroPCS) that is no longer active just sitting here and thought maybe I'd use it with my pay as you go Tmobile sim card to replace the phone the sim card is currently in which is an older basic phone. I looked at the prepaidphonenews web site which seems to indicate a phone has to be active but I could be wrong on how I read it.

 

If I cannot do what I was thinking of doing I can still use the phone as a wifi only device using google hangouts for calls here at home. The Exhibit is still in good working order.

 

Had not thought of that. Thanks!