This is most likely caused by the fact that when a Toll-Free number 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 ThinkTel 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 ThinkTel Support.
We hope this info was a help. If you have any problems or questions, please Let us know. You can open a support ticket via https://helpcentre.thinktel.ca/contact-us or you can contact us by phone at 866-928-4465 Options 1-3-1