Is there no carrier splash screen when the device boots?
Are you sure it is locked?
The info at https://en.wikipedia.org/wiki/BlackBerry_Q5
may help you. Use the model number and country to narrow down to carrier(s).
However, I gather from reading the forums that you might be better off using an unlocking service instead of the carrier. Or, that would be an option if you fail to get carrier to unlock it....If it is locked.