If your mobile phone has been purchased from your GSM operator as part of a package deal, it is likely that the Global Voice Sim card will not work in it. For using Global Voice SIM while abroad you’ll need to get an unlocked phone.
How can you tell if your phone is locked?
Insert any other operator SIM other than your own. If the telephone doesn’t allow network access, then it is locked. If it does allow calls to be made it is unlocked.
You can unlock your phone using UnlockTotalk