This is most likely caused by the fact that when a Toll Free numbers is ported away. The Toll Free provider is not required to inform the loosing carrier of a port away request.
So what happens is that if you call the Toll Free number from the PSTN, the off net calls are routing properly via the Toll Free provider.
But the number will still be in our switch, so when you try to call the Toll Free number from your Gazelle number we will try to route the call on our network and a NIS will then be returned.
If you experience issues calling Toll Free numbers. Please contact Gazelle Support.
You can open a support ticket via http://support.gazellecorporation.com or you can contact us by phone at 416-549-5000.