Each carrier has specific rules as to when and if they will unlock.

One of which is your account has to be current. If you were cut off because you were not current, you would need to find a different way to unlock it so you could use the phone with some other sim

You can also buy locked phones and blackberries on places like ebay, at tag sales, etc. In those cases, you could not just call AT&T or Tmobile to get them unlocked. If you are really lucky, you can find them on trains.

While it is possible to get a phone unlocked if you meet all the carrier's conditions, they will not unlock one if you don't.